I can't speak for other GNCs, but the one in my area is pricey compared to other stores; this is surprising, since i'd assume they had the power to buy in volume, and pass that discount on to the customer.
I have used their whey isolate, and it was fine. Pricey, but fine.
Their brand of vitamins seems average. I had a GNC employee try to tell me their brand was "top notch, and can't be beat". I brough in a bottle of nu-life brand vitamins, and showed him, and he just stared at me like this:
I won't shop there, though, because:
1) They've proven they don't really know about their products, and they try and push false facts on potentially unknowledgeable customers.
2) It's an american company.
Yes, I buy american products, but from locally-owned Canadian retailers
GNC has it's place....for those starting out, maybe, who have limited knowledge on what's out there. Once educated, though, I'd bet most poeple move on.