C32's are reliable. The engines and transmissions themselves are bulletproof. The engine is hand built (as are all AMG engines) and the transmission itself, from what I've read, is rated for up to 700ft-lb's of torque. Note that this transmission is used in the C32, C55, W211 E55, as well as some of the Chrysler SRT products - including the Jeep SRT-8. edit - I was wrong here. The Jeep uses the A580 from the previous gen W211 platform, my bad.
From what I have read the biggest problems occur specifically with the 2002 model year and potentially early 2003's with the transmissions. Well, the transmission itself wasn't the problem but there was a potential for fluid contamination from the Valeo radiator that could damage the transmission.
Other minor problems may be encountered just like any car. In general mechanical issues do not seem to be the problem with these cars. It's the little electrical demons that are the killer. The common issues you'll see are automatic windows not going up / down, seats stop moving, sunroof not auto moving. All of this can be reset just by undoing the battery for a while and reconnecting it. Sometimes the seat module may need to be replaced for seat issues.
All that said I've had my for 6 months now and absolutely love it. The power is fantastic and the ride quality is great. Haven't had a single mechanical reliability issue but I have had the seat-moving thing happen. A simple battery disconnect & reconnect reset that and corrected the issue.
So in my opinion - Great Cars! If you're going to buy one you might save yourself some trouble and get one that already has bi-xenon HID's installed. And if you ever plan on installing a stereo try to find one without the Bose option. As a point of reference I gave $24,500 for mine with 41,000 miles. I've heard of better deals as well. Keep in mind this car originally retailed for $55k+ too.
Hope that helps!
I didn't write that, pulled it from: