I have an 04 yz450f. It took some time to get the suspension and everything set up, but now it rails corners easier than anything else i've ridden. I haven't ridden the 06 yz450f and i'm sure its nice but there are a lot of people out their who always need the latest and greatest. It only has 4 gears but that never seems to be a problem. It will wheelie in last gear at the crack of the throttle and mine goes about 85mph with 14-48 gearing. I would like an 06 but even when I get on a 10 year old 250 2 stroke I still have fun. Its just up to what you want.