I've owned four BMW M5's.
1988 E28 M5
Good car, but would jump out of 4th gear under low load. Applied the recall of newer detent springs, didn't fix it, finally wrapped the car around a tree in the wet one day.
1991 E34 M5
A very good car, but it would leak oil from the timing chain cover and BMW could never fix it.
2006 E60 M5
Blew the oil cooler, brakes covered in oil, was lucky to be able to stop safely. Repaired under goodwill by BMW USA. But had a lot of failures that were expensive, such a hydraulic control unit for the SMG, idle control valves, catalytic converters. Still have this car with 150k on it, but isn't driven much.
2008 E60 M5 with Dinan 5.7 stroker
Very fast, but broke 8 rings after 20k miles. Dinan refused to repair it, demanded I purchase a new engine at his cost, which was north of 25k installed. Declined and ended up selling the car for pennies.
Now I drive my QP everyday. Other than a new clutch at 20k because of a failed pilot bearing, it's been solid, though I did put all formula dynamics goodies on it