I had to replace my clutch on the 540 a couple weeks ago. The car had a auto-manual swap done about 100k ago. I'm told that's a decent life for an M60 clutch.
One day I noticed a severe reduction in accelleration, and the rpms were shooting up-- slipping. It got worse and unfortunately I kept driving on it and killed the flywheel.
I got the car in May of 06 and only put a couple thousand miles on it since. During that time the clutch engaged higher that most other cars, and the actual engagement travel distance was rather long.
Another observation is that I can't recall ever breaking the wheels loose on a takeoff(I didn't really try to either), but since the clutch job its much more jumpy and quick to engage. I've broken then loose a couple times without thinking accidently. Its also possible that the cooler weather is reducing the traction of my summer tires making it easier for them to break, but I'm certain the new clutch has something to do with it.
Hope that helps. Bottom line, don't drive on it if you think its starting to slip. A new flywheel costs at minimum $700 which is almost as much as it it would cost for the clutch kit and install.
'94 540I 6spd/205000 miles