Occassional good deal to be found.
Pet Supply Outlet seems to have good reviews on Yelp, but I agree with the previous poster - a local, small, business with no personality. For instance, they have a parrot that talks, but don't interact with him, introduce him, or have a sign near him with any information.
I go here because they carry the brand dog food I like (thought it's spotty - they're an outlet after all), and it's relatively inexpensive. I would say on average, you save about 5% on things, but every now and then you find a really great deal on some product (like toys or leashes, I've never needed to buy grooming supplies, etc. so don't know about that.)
If you're looking for personality though, and don't mind price, I say go to Scottie Connection in Arlington. If you're looking for selection, go to Petco across the street (who is probably going to put them out of business soon anyway).
They're an outlet - they are what they are, same as when you go to Old Navy outlet and realize they have limited selection and deals that aren't that amazing.