I've ridden/raced a 2000 520 EXC in the desert for a little over a year now and have had no engine related problems yet (knock on wood). If a person can do a ring job on a 2-stroke, then he or she will have no problem adjusting the valves on a 4-stroke. I believe that the choice between a 4 or a 2-stroke really just boils down to personal preferences. I can ride a 4-stroke longer and faster in a desert race than I can a 2-stroke, but thats me. If someone is looking to choose between the 2 bikes then they should ride them both and make up their own mind. Everyone is different.