The Crankshaft Sensor & Mass Airflow Sensor were replaced @ 80,0000 miles ($600 at the dealer). Both are common with this car. Parts would probably only be $100 for a do-it-yourself type.
I'm starting to hear a slight rattle from the catalytic converter. Depending on which cat is bad, this will likely cost $500-$1,500 and the cat is only available from Mercedes.
Brakes were replaced at 70,000 miles ($300) and are still in great shape, with no noticeable fade or wear.
Suspension was replaced (shocks & springs) @ 70,000 miles to improve cornering ($800 parts/labor)
Scheduled maintenance ($2-300 per year) keeps the car running smooth.
I've tried several high-performance tires that have needed replacement in 8-10,000 miles and found the ideal tire for this car. The Bridgestone RE750 has the best dry and wet traction I've found, extremely good ride quality, and has 1/2 tread left at close to 20,000 miles.
This car received rave reviews when it debuted and my experience has been great. The 320 cabrio is a well-built machine that is everything one would expect in a Benz. It's comfortable, quiet (even with the top down and the rear windscreen up), fun to drive, and still turns heads in 2008.
Handling performance improved remarkably when I replaced the suspension with H&R springs/Bilstein shocks, which lowered the car almost 2 inches. 18-in wheels with larger (225/40/18) tires seriously improve traction over the stock wheel/tire combination. I can take a 90-degree turn at over 60 mph without fishtailing or engaging the ESP traction control (with proper throttle action).
The V6 engine is a little weak and the transmission is geared for comfort at the expense of performance. That being said, both are rock-solid reliable and perform very well when in the right RPM range and limited to 4th gear. The transmission adapts to driving style and performs well with both aggressive and docile driving.
I bought the car in 2004 for $25,000 and could probably fetch $13,000 now. That's only $3,000 in annual depreciation! I predict that high-mileage CLK Cabrios will bottom out in the $8,000 range, since there's currently a floor of $10,000.
Overall, I think that the CLK320 Cabrio is an amazing value. Total operating costs - depreciation, maintenance, insurance, and registration - have been surprisingly low (about $300 per month) and a huge improvement in performance can be realized for under $1,500.
I plan on getting a new car soon - a '03 CL55 or SL500 are the front-runners, but I might just hold on to my classic CLK as a second car. After all, in the next few years, it will only cost about $150 a month - all in.