I had one for a while -- reliability and performance were par for what you'd expect from a Glock. One failure to fire the whole time I had it, and that was definitely the ammo, not the pistol (good, deep primer strike, just no ignition).
Accuracy seemed good as well, though I find I have to do a lot of shooting with a Glock trigger to get really acceptable groups with them. It's not the pistols, definitely me, but the Glock trigger is different enough from other stuff I shoot (1911s, CZs, Sigs) that going from them to the Glock opens my groups up a lot initially.
The only real downside with the Glock 21 is the grip size. It's a pretty big grip, and some people don't care for it. Others don't mind it at all, or even prefer it. I'd recommend checking it out and seeing how it feels in your hands, how well you can get to the controls, etc. If the ergonomics work for you, you probably won't have any other issues with the pistol at all.