I beat the piss outta mine just about everytime I ride it...i've had relatively few problems so far. As far as a normal 50+ miles ride a day goes i've had nothing that really ruined my day. The oil blowback thru the airbox was the first thing i encountered that worried me, but a simple kit from Hunterworks resolved that problem. My stepdad, on the other hand, has the 3rd or 4th 09 Teryx produced and hes had a few problems from it. He broke a belt (due to lack of adjustment), had a bit of a freak accident with the front end exploding, well the diff-lock side, but no one we've talked to has seen that happen with another 'rex. Knocked a few tires off the wheels, filled the engine with water, stuff that was more rider error than kawasaki error.
For comparison purposes we ride with a bunch of Rzr's and Rhinos, I've seen more problems from them than anything. Rhinos eat axles, and rzrs eat axles and belts. A friend that runs the service dept of the local Honda/Suzuki/Yamaha dealership says he sees more Rzr's in for problems than anything. I had originally wanted a Rzr from riding with several and seeing how they out performed the rhinos and such...but after riding with them more and more and seeing they were a constant problem I opted for the Teryx; I also couldnt buy a used Rzr that wasnt beat to hell and back for what I paid for the Teryx.
I'm running an 08 stock other than Seats, Wheels, and Tires...I've broken 1 axle on the front but I was pulling HARD in reverse with Diff lock engaged and against another dead teryx that had backed off into a hole in a Creek. I also broke 2 winch cables and a 3" strap that day trying to get the thing outta the hole, but it finally came out and I had to tow it back to the truck on 3 wheel drive. And the other teryx was dead from backing into the hole in the creek and filling the engine up with water...so again, rider error.
As far as performance, I couldnt be happier for a stock machine to perform the way mine does. I'm chomping at the bit to throw some performance parts on it and see what it'll really do. Same goes for Reliability. I've never had a problem with it starting or pulling or anything that would make me question riding it. *Knock on wood*
Apologize for the long winded post, just trying to be as informative as I can with my experiences with these things...