The original hybrid automobiles were good for the planet and great for your conscience, but they didn’t exactly provide a rush of adrenaline when you hit the accelerator. They offered competent but drab transportation for the übergreen set. Now, a growing number of luxurious, planet-friendly models cater to tree huggers in tuxedos who prefer topflight appointments and have the means to pay for them.

MoneyWatch.com road-tested three luxury hybrids — a Mercedes, Lexus, and Cadillac — and came away pretty impressed. (We’re also telling you about the Fisker Karma and BMW X6 ActiveHybrid, although we haven’t been allowed to drive them yet.)

Two big caveats: First, you’ll pay a stiff premium over non-hybrid versions. Also, because these are big, safe luxury cars, the added weight of such things as batteries, massive engines, and heavy doors that go clunk generally drags down fuel economy. So don’t consider one of these beauties if you’re hoping for bragging rights about superior mileage. Of the five, only the Fisker Karma (a plug-in that runs partly on electricity) is expected to improve on the fuel economy of the humble $15,000 Toyota Corolla (26 mpg city/35 mpg highway).

Before getting to the reviews, a short course on hybrid technology. The cars employ three types of hybrid systems: Mild hybrids (Mercedes and Lexus) use their electric motors just to provide extra power for accelerating and high-speed cruising; they never run on battery power alone. Full hybrids (Cadillac and BMW) have larger motors and battery packs; they can start out in electric mode, before the gas engine kicks in. Plug-ins (Fisker) have bigger battery packs, the ability to recharge from a wall outlet, and offer 30 to 50 miles of all-electric cruising.

2010 Mercedes-Benz S400 Hybrid

Lowdown: Mercedes could be accused of creating its hybrid as an afterthought, responding to American market demands. The company’s heart is plainly in the diesels U.S. consumers stubbornly refuse to buy. But the sleek 2010 Mercedes-Benz S400 Hybrid has the manifold virtues of Mercedes’ big sedans — effortless high-speed cruising, top-quality interior design, and cutting-edge tech features. Consider it a car for executives whose kids (or grandkids) have been bugging them about environmental responsibility. It’s not a major departure from the company’s non-hybrids, but a supplemental electric motor gives it modest green credentials — 26 mpg highway is better than most luxury hybrids.

Performance: Under the hood, a fuel-efficient 275-horsepower V-6 connects to a seven-speed automatic transmission. Its relatively small 15-kilowatt electric motor adds power under heavy loads, restarts the engine and — after the car shuts off at stoplights — aids in pulling away. But the electric motor never actually drives the car. This five-seater is the first hybrid car with a lithium-ion battery pack, which promises improved power and less weight than nickel-metal-hydride packs. The hybrid’s 7.2-seconds from zero to 62 mph is roughly comparable to the S400 V-6.

Luxury features: The S400 is the starter-level model in the “S” Class, but it comes fully equipped. A facelift adds new design elements, LED lighting, and a host of high-tech safety features, including headlights that automatically dim for oncoming traffic, lane-departure warnings, and driver drowsiness monitoring. In keeping with its green theme, 85 percent of the materials are recyclable, and 45 components are made from recycled plastics. But don’t worry: the interior trim wood veneer is authentic.

Price: $87,950

Fuel economy: 19 mpg city/26 mpg highway

Drawbacks: It’s heavy — at more than 4,500 pounds — and pricey.

Reviewers say: “For 2010, the Mercedes-Benz ‘S’ Class gains its first-ever hybrid powertrain, a clear indication that its maker is finally acknowledging the worth of hybrids as an essential step on the path to electric propulsion.” — Automobile Magazine

2010 Lexus LS 600h

Lowdown: This is the ultimate luxury hybrid sedan, a performance mothership so high-tech that Captain Kirk would pilot it with approval. The 2010 Lexus LS 600h is fast and almost effortless to drive, with a hushed cabin that lets you listen to the 19-speaker stereo with minimal intrusion. Adapted from the non-hybrid LS 460, the six-figure-priced LS 600h is now offered as a long-wheelbase “L” model, with five extra inches of rear legroom. (We road-tested the 2009 model, which differs in minor cosmetic ways from the 2010, but the driving experience is essentially the same.)

Performance: The five-liter V-8 starts with 389 horsepower, but the electric motor pushes output to 438 (more than twice that of the four-cylinder Toyota Camry). Switch to “Eco” mode and performance is slightly more modest but fuel economy improves.

Luxury features: It would almost be easier to list features not on this car. It comes with an audiophile-quality Mark Levinson stereo, voice-activated navigation (with live traffic updates), heated seats that also offer a built-in massage and reclining option in the back seat, plus a rear camera to avoid backup accidents. Options include sci-fi gizmos such as an infrared skin-temperature sensor system to automatically adjust heat and A/C. The LS 600h’s Advanced Parking Guidance System can also tuck the car away for the night, steering and manipulating the gas pedal. Pull up parallel to a spot, put the car in reverse, and the computer control takes over, with sonar to measure the distance.

Price: $106,035

Fuel economy: 20 mpg city/22 mpg highway

Drawbacks: The price is outrageously high, especially for fuel economy in the low 20s. If the LS 600h is too rich for your blood, consider the $34,200 Lexus HS250H (34 mpg city/35 mpg highway), a near-luxury version of the Toyota Camry Hybrid.

Reviewers say: “The Lexus LS 600h L is the grandest hybrid sedan on the market, with some of the world’s most advanced technology features and luxuries that rival those in top German sedans.” — The Car Connection

2010 Cadillac Escalade Hybrid

Lowdown: As the ride of choice for Queen Latifah, Shaquille O’Neal, Ludacris, and Justin Timberlake, the seven-seat 2010 Cadillac Escalade Hybrid has won a starring role in many hip-hop videos. This is also a car for those who travel with the Cub Scout pack in the 90210 area code. A behemoth that’s more at home on city streets than in the backcountry, the Escalade Hybrid marries GM’s SUV brawn (it’s a Chevy Tahoe hybrid under the sheet metal) and Cadillac’s upscale appointments. Its new hybrid system, a major advance from the mild electric assist in previous models, improves fuel economy 50 percent. But hold the trumpets. We’re talking about city mileage moving from a dismal 12 mpg to a so-so 20. It’s hard to make serious environmental claims for a car that takes up so much space and returns such marginal fuel economy. Still, the Escalade drives well (we road-tested the 2009, which performs exactly as the 2010 will), although parking can be a chore.

Performance: The 332-horsepower, six-liter V-8 engine is connected to two 80-horsepower AC electric motors and a 300-volt battery pack located behind the second row of seats. Cylinder deactivation can shut off four cylinders at cruising speeds to further conserve fuel. Like most full hybrids, the Escalade starts out in electric-only mode if you accelerate gently, but its range on batteries is only about a mile.

Luxury features: The base car is well equipped, with such features as remote start, a power liftgate, and a rearview camera. But Cadillac pumps into the Platinum edition (costing $12,000 more) just about everything on the base car’s option list, plus a full set of LED headlights that last 20 times longer than halogens, and a rear entertainment system with a pair of screens in the front seatbacks. GPS navigation is standard in the Platinum, as is magnetic ride control, which automatically adjusts to road conditions.

Price: $73,425-$75,975 (base); $85,225 to $87,725 (Platinum)

Fuel economy: 20 mpg city/21 mpg highway

Drawbacks: Awkward in city driving, not particularly fuel efficient, and expensive

Reviewers say: “Fuel mileage is better than the regular Escalade’s, but the price premium is pretty high.” — Car and Driver Buyer’s Guide

2010 BMW X6 ActiveHybrid

Lowdown: The 2010 BMW X6 ActiveHybrid, BMW’s first full hybrid, hits showrooms in December. It’s an SUV that’s not really intended for off-roaders; think of it as more like Porsche’s high-performance Cayenne with a utility bonus. BMW will offer the ActiveHybrid 7, a mild hybrid version of its big 7-Series luxury sedan, next spring.

Performance: The dual-mode ActiveHybrid system — a close cousin of the Escalade’s — consists of a powerful twin-turbo, 400-horsepower V-8 engine, two electric motors (producing 91 and 86 horsepower), and a nickel metal hydride battery pack under the luggage floor. This BMW starts running on one electric motor and when more power is needed, the second starts the gas engine, and then acts as a generator to supply electric power. With a possible 480 horsepower, the X6 ActiveHybrid is no slouch on the road. It can hit 60 mph in an astonishing 5.4 seconds. The car can also reach 37 mph in full-electric mode, but its electric range is limited to 1.6 miles.

Luxury features: Like the Mercedes S400, the BMW X6 ActiveHybrid features advanced safety features, including crash-activated headrests. Twenty-inch light alloy wheels are standard, as are bright Bi-Xenon headlights.

Price: Not finalized, but likely to be in the high $80,000s

Fuel economy: BMW isn’t saying yet, but expect an approximately 20 percent fuel-economy boost from the non-hybrid V-8 or roughly 18 mpg combined city/highway.

Drawbacks: The X6’s boast that its carbon dioxide emissions are 20 percent less than those of a regular X6 is less than meets the eye. As the Guardian has reported, 231 grams per kilometer is double the emissions of a Toyota Prius.

Reviewers say: “On the scale of political incorrectness, the BMW X6 contends for the top slot with such behemoths as the Porsche Cayenne, Hummer H2, and Cadillac Escalade. ... The bad-boy BMW makes its case by being not only huge, but loud, thirsty, unconscionably fast, and the least practical of the bunch, too. ... With what promises to be a mountain of torque available from idle, the hybrid X6 may prove as entertaining to drive as its conventional brethren.” — Car and Driver

2010 Fisker Karma

Lowdown: Fisker, based in Irvine, Calif., is an ambitious startup with 32 dealers that will start selling its high-performance hybrid in 2010. Fisker is currently taking $5,000 deposits to preorder the Karma ($25,000 for the Karma S convertible), and anticipation is running high for the 2010 Fisker Karma sedan, which is as curvy as Megan Fox. No surprise: Al Gore is on the waiting list. The Karma promises an intoxicating package of exhilarating driving, sumptuous comfort, and environmental performance so stellar (67 combined mpg!) that Greenpeace members could drive it with pride. Target audience: price-is-no-object buyers in search of coolness and green credits.

Performance: The home recharge will give the Karma a 50-mile range on pure battery power. When the battery runs out, its Q Drive system kicks in with a turbocharged two-liter, four-cylinder engine that acts as a generator to supply the electric motor and lets the car go another 250 miles. You’ll get more than 400 horsepower when the Karma’s gas engine and electric motor combine to hurl the car from zero to 60 mph in less than six seconds.

Luxury features: A solar roof makes a modest stab at recharging the batteries; you can also use the solar panel to run the climate-control system. The luxurious interior is gilded with social responsibility: The “EcoChic” model is animal-free, leather is replaced with soft bamboo viscose and the center console is made of layered EcoGlass framing fossilized leaves. If you still eat meat, order the “EcoSport” and get leather seats (sustainably produced, naturally).

Price: $87,900 (minus a $7,500 federal tax credit for battery cars)

Fuel economy: The company says the Karma will have a combined 67 mpg equivalent, with an operating cost of just three cents per mile.

Drawbacks: Experimental technology, plus as-yet-untested quality and dealer support. It may take a while to get one. Put down $5,000 now and your car is likely to be in the driveway next May.

Reviewers say: “The Fisker Karma — also known as the most badass hybrid sportscar you’ve probably never heard of ... The unofficial dream car of Earth Day.” — Esquire

More on MoneyWatch:

• Cadillac Escalade and Lexus: Hybrid Reality Check
• 6 American Cars Worth Buying Now
• Tesla Test-Drive: Time to Try an American Car
• 6 Big Myths about Gas Mileage
• Skip the Hybrid ... for a Diesel?
• The Best Time to Buy a Car Is ... Now
• The Perfect Car for a Teenager