Bought my 2012 850 Touring earlier this year. I've put about 200 brutal miles on it without a single issue.
Before I bought it, I took my buddy's 550 Grizzly for a 30 mile ride and that thing beat me to death. I took another friend's Sportsman 550 on the same ride a week later and I was hooked on Polaris. Everything felt more refined and more stable on the Polaris. I hated having to lose my momentum, push a button, flip a switch, and then be in "real" 4wd on the Grizzly. The Polaris AWD/4x4 system completely sold me, no need to worry about how fast or slow I'm going, no need to worry about having to push another button to get all the wheels spinning, just go. When you need all 4 wheels, they're all putting power to ground, no hesitation or worry. The smooth ride of the Sportsman convinced me further; Polaris is where it's at.
I also really like playing around in ice and snow on my 850. The engine braking system helps you drift the rear end around pretty handily if you've got some speed going into a corner or doing a 180º turn on a dime. Just let off the throttle and bump the steering a tad and you'll swing around, it's really fun.
A couple of weeks ago, I had to tow my buddy's 550 Grizz (he swamped it), my meat wagon, and his meat wagon (small, single-axle, high-wall trailers with ATV tires used to transport killed/caught game and gear off-road) about 15 miles back to the truck. I put the 850 in low gear and hauled it back without a single issue.
Another friend of mine needed his old Dodge Ramcharger towed up the driveway, but there wasn't any room for a truck... I hooked up my 850, dropped it in low gear and hauled that 3-ton beast up the driveway. No fuss, no frills, just a machine that works when I need it, does anything I ask, and begs for more.
I'm sure I'll have problems with my quad, just like any other quad, regardless of brand name. It's part of it, and I enjoy turning a wrench every now and again. Keep the fluids topped off, grease the zerks often and with quality grease, and don't exceed your limitations. I do not regret buying my 850 Touring, and when it's time to buy a machine for the wife, the only machines I'll look at are Sportsman XP's.