I just had an exhaust valve "burn out" on a 60,000 mile '93 R100GS-PD. Before then it was running a bit rough, and had started using more petrol (below 40mpg). The valve had an interesting green plastic like deposit on the underside.
The valve was replaced and the seat reground by a BMW dealer. They also suggested that i use a petrol additive designed for leaded heads, as they suspect that the 2-valve unleaded models may still have problems. It's now running very well, starting without choke, idling correctly, and using less petrol (46mpg).
I am cosidering getting the remaining valves overhauled soon.