The engines on these things are bulletproof. Just change the oil at regular intervals, check your valves every once in a while, and don't totally beat the piss out of it and you'll never have any trouble with it. Heck, usually if you DON'T change your oil, DON'T check your valves, and DO beat the piss out of it they're still reliable engines.
It seems like the most common problems with these machines are electrical-related, but in the 3-4 years i've had mine I haven't had a single issue electrically, even riding in mud and water often. I think the most annoying problem with mine (aside from things that were problems due to my own dumb fault, or bad aftermarket parts), was that I spent a lot of time replacing bushings and bearings all over the quad.
If you're hard enough on it you might discover that the frames aren't the strongest ever built, but this usually only comes to your attention after crashing into stuff at high speeds.
Other than all that, just look out for that stock air filter setup. The way it's design utilizes the lid and a foam gasket to hold the filter in place and seal out dirt and other junk. If you don't get the filter and lid lined up just right, or if that foam gets torn or comes unglued, you'll be sucking dirt into your engine and probably won't know it until the next time you take your airbox lid off to clean the filter, and by then the damage is done.
To minimize this risk I suggest that you would get a clamp-on filter kit from Pro Design. With this kit you can still keep the airbox lid if you so desire, but the filter will be securely clamped on and sealed instead of the iffy stock design. Then if you want to leave the airbox lid off and rejet your carb to get a little more hp you can do that also.
