I have owned this car for for about a 1.5 yrs now. The only problem I had with it was a speaker in the door going out and I had to replace the hood pad that was falling apart. Other than that nothing at all. I bought it with 78K in great condition. A very fun and great looking car, lots of power and with the windscreen up you can hardly tell the top is down. The only negatives are the rear window creasing and the poor lighting of the non-xenon headlights, other than that this car is a dream to drive. I am very happy with the SL500, like poetry in motion, enjoy the drive, this is a one in a million automobile for the discriminating owner. With the prices of a used SL it is a real bargain.
The windscreen, Bose audio system, great interior quality, climate control system. Handling is superb and the ESP system works great as well.
Xenon headlights are a definite plus. The rear plastic window is not very durable, I know this is an old model now, but if you find one with a new or good condition top/rear window take good care of it. As for improvements to this model, it is a little late to mention now.