Eh. When I was in my early stages of this hobby, I got a few fish at Petco. I'd say about half of my Petco fish died within a day or two. The rest...well... A pair of firefish, two pajama cardinals, a coral beauty, a kole tang, a sixline wrasse and a sailfin tang, are still with me, years later!! As for fish that I bought at my "real" LFS, the only deaths I've had of those guys were jumpers, and I bought fish from them from the beginning too. So there's DEFINITELY a difference in quality/health of the fish in my area Petcos, at least.
And most of those Petco fish I bought on impulse because their prices were mismarked!! I think I paid $15 for the kole and sailfin tangs.