I ran my 05 from new for nearly a year without problems, the problems happened when i put a hole in the side and lost all the oil. Lucky for me it nipped on the piston and stopped the bike before it did any real damage. After sending the head away and getting the seats cut and new valves installed it again ran for a fair period before they started to move.
I now run the bike give it the odd shim just to assist and as soon as the valves start to move alot get a new set put in. the cost is not that high. I may think about fitting stainless or getting a set of better titanium to try and will probably replace the seats next time.
All in all the bike is a pleasure to ride and if you race the things you have to expect to spend money. We used to race supermoto Quads with KX500 engines these required full strip and rebuild (gearbox inc) new piston and even had to replace the engine studs after 6 weekends of racing. That said the kx was giving 75bhp at the rear wheels of a quad.
Thanks to Honda for building this excellent little bike.