I actually find the fish from PetsMart to be pretty healthy. Certainly much healthier than fish from Wal-Mart.
I don't buy fish from PetsMart for a couple of reasons. First, if PetsMart can get it, lots of LFS in my area can / do stock it already (and probably for the same price or less). I prefer to support a good LFS than a chain.
Second, they simply don't stock many fish that I want.
Finally, there isn't always enough staff or enough knowledgable staff to provide good service at PetsMart.
Wherever that you buy fish, it's important to QUARANTINE. I know everyone doesn't have 50 tanks...but an extra tank is all you need.
Matt