My '96 RT has 176,000 miles on it. It's been a wonderful mount.
Engine has been nearly perfect; burns no more oil than when new (About 1/2 quart between 6k changes on petroleum oil). I think the basic engine will go forever.
I do all work on it myself, including maintenance according to the book.
Has these issues:
- Trans shifting has deteriorated since 80k: now slips out of 3rd sometimes during upshifts. But, I can live with it, and it's quite expensive to have corrected.
- Hall Effect ignition sensor failed at 135k in Nova Scotia. This was BMW's fault for not advising to clean the sensor area (behind the lower alt.belt pulley), as road dirt gets in over time, making an ugly slury, which can short out the sensors when rain water gets in, which it will, as there's no effort made to keep it dry.
- Clutch is original. Yes, abuse can kill it in a hurry, but as I ride one-up always, and am careful to go easy on it (no fast starts), It's been reliable.
- Rear end has been faultless. Change the oil as directed.
My suggestion is to be careful to buy a bike that has had careful maintenance. You can tell a lot about a bike by checking it carefully and asking for maintenance records. And, talk to the dealer who does the service.