I've had a '96 MB SL500 for four years. It now has about 68,000 total miles. I've had no serious problems at all during the time and 8,000 miles I have put on it during the summer months. The V12 SL600's seem to have a few more issues and not that much more performance.
One of the biggest issues that seem to crop up with the R129 platform has to do with the hydraulic top mechanism and the seals for the many cylinders that control the top. One day everything is fine and the next you have a puddle of hydraulic fluid on your seat and floor. It hasn't happened to me yet but it's just a matter of time.....
The dealer cost for repairing/replacing these is outrageous. However, there is a very straight forward DIY fix that involves sending the cylinders out for rebuilding. Check out the BenzWorld forum for lots of information on these cars:
http://www.benzworld.org/forums/r129-sl-class/
Other issues involve electronics and various electronic modules. Dealer pricing can be steep.
Overall I like the car for what it is. It's not a 911 or a 928. As I tell people, the Benz is really enjoyable to drive while the 911 is fun to drive. There is a big difference.