The Mercedes-Benz CL550 we tested is a swift and smooth ride to be sure, but we'd stop short of calling it a sports sedan. It's simply too large, too soft and too luxurious. But it is rewarding to drive for just those reasons.

You start the CL with a touch of a big aluminum button to the right of the steering column. We still wonder why being able to keep the key in your pocket makes this a better solution. Then drop it into gear with a new column-mounted electronic shift lever similar to the ones BMW is now using. Purists may feel it's an odd and un-sporty throwback to have a shifter moved off of the center console and on to the steering column, but it works well and frees up space.

The 5.5-liter all-aluminum 32-valve V8 is velvety smooth and nearly silent, until you prod it. With 382 hp on tap it rushes the car to speed with a muted, purposeful growl. (Mercedes quotes zero-to-60 mph in 5.4 seconds.) The seven-speed automatic gearbox shifts imperceptibly in town, smoothly at full throttle and never gets caught in the wrong gear in traffic. Quiet, smooth, sophisticated – this is the way the powertrain in a high-end luxury automobile should behave.

Having a gasoline-fired engine this powerful pulling a 4360-pound vehicle does create a gas mileage penalty, two actually. The first is real-world fuel economy: the EPA mileage rating is 15/22 City/Highway. And that figure triggers the Federal Gas Guzzler Tax at purchase, $1300 in this case.

If there's one word that describes the CL road experience, it's "silken." On smooth surfaces it feels as if it's riding on glass. Some vibration or road harshness must be penetrating the hushed cabin, but it just doesn't feel like it. The sportier BMW 6 Series coupes register bumps harder and reveal surface imperfections far more acutely. In the Benz, the smaller road irregularities get glossed over. Over larger bumps the ride is less supple than you might expect, almost firm, but not enough to inspire the driver to attack the curves.

The steering has a ball-of-silk feel, less sharp than the BMW and more relaxed in its responses. Though the steering effort rises with road speed, the feeling remains comfortable, smooth and luxuriously isolated rather than sports-car sharp. This is a car that wends its way down a winding road with grace and stability; the active suspension keeps it cornering quite flat. But the CL doesn't communicate the sense of the road in the way that great sports sedans do. It never gives you the urge to get aggressive like a BMW 3 Series would.

On the highway, the CL's German DNA is fully in evidence. It has a commanding, solid feel and is dead stable even at extra legal speed. It's in these upper speed ranges that you notice that wind noise has hardly increased at all. This is autobahn breeding at work.

Using the optional Distronic Plus distance sensing cruise control is an eerie and fascinating experience. The radar-based distance monitoring system automatically slows the CL, using the brakes if necessary, as you close the gap on the car in front. That distance can be set between a hundred and several hundred feet. When the system detects the lane ahead is clear again, it accelerates back to your pre-set speed. All the driver needs do is steer, an odd sensation to say the least. The system works beautifully in light Interstate traffic and reasonably well in moderately heavier intra-urban highway traffic, though it sometimes annoyed us by slowing sooner for a car up ahead than an average driver would in most circumstances.

There's more to Distronic than active cruise control. The system is now tied into a comprehensive in-car safety network. Distronic will sound an alarm if the driver is gaining too fast on the car ahead, meanwhile priming the Brake Assist Plus system to apply full emergency braking as soon as the driver presses on the brake pedal, no matter how lightly it's applied. If the driver doesn't respond to the distance alarm, the system will apply up to 40 percent of total braking capacity automatically to slow the car down.

Meanwhile, if a frontal crash is imminent, the Pre-Safe Brake system takes action: it tightens the front seat belts milliseconds before impact; moves the front passenger seat to its safest position, rams the side windows closed to add support for the side-curtain airbags (and to keep occupants arms inside the vehicle), initiates partial braking to slow the vehicle and will even close the sunroof in a rollover.

We found that in normal driving, the CL's brakes were confident, effortless and luxuriously insulated. The brake pedal action is progressive and direct. Front brakes are ventilated and cross-drilled rotors clamped by hefty four-piston calipers; the rear brakes utilize single piston calipers. You won't find a smoother set of brakes anywhere. In hard braking the system feels powerful and was free of any fade. Decelerations from even high speed were calm, quiet and drama-free, with not a whit of vibration or noise transferred through the brake pedal or into the cabin. Again, thank the requirements of German autobahn driving.

We found using the COMAND system while underway distracting, but we didn't have much practice. It is complicated enough that it will take an owner a period of time to absorb the combination of button-pushing and knob-twirling-and-tapping that best accesses and adjusts the CL's many features. Exploring the system while on the road divides the driver's concentration. In our weeklong test session, we found it best to slow down, pull over into the right lane and stay out of the way while fiddling with the system. We figure at least a month would be required for an owner to fully master COMAND, maybe more.

Virtually all of what we reported on the CL550 and its multitudinous systems is true of the CL600, which includes virtually all of them as standard. While we didn't sample a CL600 for this test, we have driven enough twin-turbocharged V12 Mercedes to know that the major difference will be increased power and even greater, whisper-smooth engine smoothness. The CL600 delivers 510 hp and, even more important, 612 pound-feet of torque, an astounding 56 percent increase over the CL550.

The V12 Mercedes we have driven in the recent past are so smooth and quiet in stop-and-go traffic they feel almost like silent-running electric vehicles. Yet awe-inspiring acceleration is just a push of the pedal away; Mercedes quotes a zero-to-60 time of 4.5 seconds, smack in the middle of the range for Corvettes, Porsches and Ferraris. There's so much low-end power on tap that the tires would spin wildly if not for the traction and stability control systems working overtime. Highway acceleration feels like a DVD on fast-forward. We don't know why anyone would actually need this much power in a CL, but it is amazing to experience it.