I've had my 05 RMZ 450 since March 05. It has 64 hours on it with no problems. It would have more hours but I took two major soil samples last year that put me on the sidelines for about 6 months (First injury, my competition took me out, second injury piliot error :bang: ). I change the oil every 3-4 hours, oil filter every other oil change and air filter every ride. The valves are checked every 12 hours and the cam chain is adjusted. Valves are still in spec, however I expect the intake valves will need shimmed the next time they are checked.
Over all the RMZ 450 has shown to be very reliable. There have been some reports of the cylinder walls flaking on the 05 that results in main bearing and or top in damage also. However, these reports have been small in numbers.
Let the suspension break in, it takes about 3 hours, set the sag at 105 mm, put a fuel screw in it and have fun. I also changed gearing from a stock rear 49 tooth to 50. For my local track this allows me to use third in all but the slowest corners and fourth down the longer straights. For me, this set up works well. I love this bike as it made riding fun again! ;)
Heres a link to my write up when I first bought the bike.
http://www.dirtrider.net/forums3/showthread.php?t=113376