These cars are all based off the E-Class. They are essentially 4 door coupe versions of the E-Class. It's six inches longer than the E-Class, but still smaller than the S-Class. It was intended to compete with the BMW 6-Series.
The engines on the CLS range from a 3.5 liter V6 to a 6.2 liter V8. There is also a V6 diesel engine offered in some markets. The transmission is the 7 speed automatic being offered in many new Mercedes'.
The CLS 500 and CLS 55 AMG are both sold in the United States. They retailed for $65,620 and $87,320, respectively, in 2007. A higher end CLS63 sells for $92,500. That car comes with F1 style paddle shifters and 19" wheels standard.
About 10,000 of these cars are sold in the United States each year. That makes them less than rare, but still not common. If you plan to buy one, you can either go to your Mercedes dealer, or you might find a used one if you browse the eBay listings.