I've noticed a few things with reverse.... the first that I noticed was that when shifting back to "forward" you have to mess with it a little to sort of latch/lock into place. If you don't it seems to pop out and put you in a sort of wierd "neutral state" that grinds like ws350 mentioned above.
The second thing that I have noticed is that sometimes when switching to reverse while in Neutral, the quad will start going backwards without giving it any gas, gotta make sure that I have my foot on the break. Mind you this is with the clutch pulled in. Not sure if it's a clutch adjustment or something else, but I have experienced this.
The one other problem that I have had, only occurred once so far. I was sitting in first gear, shifted to reverse and backed partly up a hill, and then tried to shift back into forward. It wouldn't get out of reverse, it was locked there. Not sure if I pinched something or what, but after messing with it for a couple of minutes (five or so) it finally went back to forward. Ever since then I've been shifting to reverse only with the tranny in nuetral.