My Dad has a 1979 450SL, and is seriously considering an R129 SL500 since the prices have fallen like stones since the R230 came out. There are SO many nice traded SL's, this is a super time to buy.
He's being doing tons of homework on the cars, and talked to as many owners as possible.
Things he's gleamed:
The SL500 and 320 use the "tried, tested, and true" drivelines from other MB products, so there is no "unique" engine problems or anything like that. The M119 5.0L is bulletproof, and nothing seems to go wrong with it.
He talked to a high percentage of owners with AC problems, and the fixes were costly. But, once repaired, they seemed to hang on longer than from the factory.
It seems just about everyone had some minor soft-top problems, but nothing serious when considering the complexity of the thing.
Brake pads/rotors were REALLY short lived, nothing new for an MB, but I guess the high weight of the car means more brake wear.
Overall, all owners seemed really impressed. Most said they were trading for the R230. Not that they wanted rid of their car, but they liked the new look, and the integral hard top was a big selling point.
TO a person the thing they liked least were the dealers... (nothing new in these parts1)
1998 C230 "Black Betty"