While I would be cautious, I wouldn't be so quick to give all Petcos such a bad rap...
IMHO I have 4 dogs, and think that it has to be judged on an individual basis. Right now I'm using a wonderful groomer at a Petco after going through a ton of independent groomers that didn't suit me for one reason or another.
Granted she came highly reccommended and is the only one that I leave my dogs with (like any quick-stop human beauty/barber shop they seem to have high turn over!) and she does the best job yet.
I can also say that I left in tears and cancelled a check at another Petco because of the way they hacked-up my Marley right before I had him scheduled for professional pics...
If I thought I could do it myself, I would. But my poor babies would be the ones in tears if I tried to cut their hair!