The drive-by-wire brakes, on the other hand, are not so satisfying. They will stop the CLS right now, and repeatedly, with no noticeable degradation in performance. Yet the engineers haven't quite figured out how to make these brakes more pleasing to the senses. The electronic brakes lack the steady pedal feel of the best mechanical systems. It's more difficult to modulate the pedal consistently, and smooth stops befitting a car of such grace can be a challenge.

Electronics aside, the CLS63 AMG stops like a sports car, and goes with almost as much enthusiasm as a race car. Its 6.2-liter V8 is a welcome improvement. Previously powered by AMG's supercharged 5.5-liter V8, this car with was fast. With the new 6.2-liter V8, it's blindingly quick but it's also super smooth. With 507 horsepower, the naturally aspirated 6.2-liter V8 delivers slightly more power than the supercharged engine it replaces. The 6.2-liter engine gets racing-style features such as dry sump lubrication, which ensures proper oiling under extreme g-forces.

This is the first AMG engine matched with the Mercedes seven-speed automatic transmission. Called, the AMG Speedshift 7G-Tronic, it pairs with the 6.2-liter V8 to form one of the most impressive powertrain combinations from Mercedes in years. Throttle response is instantaneous, automatic downshifts nearly so. The manual control electronics, which allow a driver to shift the transmission like a manual using either the shift lever or paddles on the steering wheel, are excellent. Speedshift allows the driver to hold a gear right at the 6.2 V8's 7200-rpm redline, where it's much silkier than the previous supercharged engine. There's a nice burbly exhaust tone at part throttle, and something more like a rebel yell when you floor it.

In the CLS63, the air suspension is tuned with a bias toward the handling side of the equation, or more firmly than some owners might like. It's also fitted with larger, Z-rated tires. In our view, it's not uncomfortable, and the payoff in improved response is worth it. Most Mercedes-Benz cars will go along for the ride when pushed, behaving predictably. The CLS63 AMG likes to be pushed. Think of it as a more willing participant in a spirited drive. SCCA Trans-Am champion Tommy Kendall gave took us on a couple of laps around the fast 2.5-mile Willow Springs International Raceway and quickly showed that that CLS63 worked at the limit of its tires very well at high speeds. Kendall had little trouble changing direction with a quick lift of the throttle while sliding the CLS63 through the turns.

Summary

The Mercedes-Benz CLS is beautiful to look at and almost as pleasing to drive. It gives up one seat in passenger capacity to most luxury sedans, but its visual impact is undeniable. There's plenty of power from the CLS550 with its new V8, and the CLS63 AMG is a real screamer.

NewCarTestDrive.com correspondent Greg Brown filed this report from Rome.

Model Line Overview

Model lineup:Mercedes-Benz CLS550 ($66,900); Mercedes-Benz CLS63 AMG ($92,200)

Engines:382-hp 5.5-liter V8; 507-hp 6.2-liter V8

Transmissions:7-speed automatic

Safety equipment (standard):front multi-stage airbags, front and rear passenger side-impact airbags, front and rear curtain-style head protection airbags, seatbelts with pre-tensioners and force limiters, Electronic Stability Program skid-management electronics, ABS with Brake Assist, roll-over sensor, low-tire pressure warning system, Tele-Aid emergency telematics

Safety equipment (optional):N/A

Basic warranty:4 years/50,000 miles

Assembled in:Sindelfingen, Germany