There have been some documented cases of the zinc alloy (zamak) slide cracking. The majority of these seem to be with the 5 inch "target" model though some of the 3.5 inch have cracked as well. I have around 3000 rounds through mine and I've found it to be plenty accurate for plinking and mine has not been ammo sensitive at all. In fact, its been ironically one of the most reliable 22 auto loaders I've owned. You'll read a lot of complaints about the P22 but mine so far has been a real gem.
Now having said that, there is a new competitor to the P22 on the block. The newly introduced Ruger SR22 pistol has an aluminium slide and seems to be more solidly built. (I've had one for a couple weeks now). Its priced lower than the P22, is backed by Ruger customer service, is made here in America, and mine so far has been very reliable. Accuracy wise its about the same as the P22, at least in my hands. Definately worth considering if your in the market for a small "combat" style 22 pistol.