I test drove a 2006 black C55 in pristine condition. The car only had 17K miles. I like the way it pulled so strongly, the glorious exhaust and the handling feel. Interior is tastefully done except for Merc's silly cell phone buttons on the center console which they still have in the current C class models. In terms of looks, I actually like the previous generation C class than the current one. The C55 looks smaller, more fluid and sleek than the C63.
I drove the car on a Saturday and was going to put an offer the following Friday but found out it was already sold! So I visited a Lexus dealership the next day, saw the IS-F and the rest is history!
I did research the C55 and 2 things you should watch out for is signs of oil leaking under the valve cover and HVAC stepper motor. The HVAC motor has flimsy plastic gears which tends to break causing loss of control of temp/airflow. It's a costly repair requiring diassembly of the dashboard. The telltale sign is a clicking sound behind the dashboard when the fan is on.
Other than that the V8 is much more reliable (not Lexus/Toyota reliable but not bad for a European car) than the supercharged V6 on the C32. If possible you should have your mechanic check the car before buying. Keep in mind that there will still be more maintenance and unplanned trips to the mechanic vs the IS.