I now have 128K on my 95 S420 and have been really fortunate not to have any problems since I bought it at 101K last year.
I had a scare with the evaporator earlier and I thought there was the noted "big problem" associated with it in earlier years of our model W140, but a couple of cans of 134a with sealant fixed the problem.
I did not purchase the extended warranty because most only offer them for cars with less than 100K miles. So far, the car is running great and has no problems. My hope is that it remains that way for a long time. As a preventive measure I change the oil often and do regular maintenance religiously.