Most of the time you won't find the best prices at any given GNC, however every once in awhile a sale will roll around that you can't ignore. Whether it be Groupon aided or even a store sale, chances are your wallet will win.
It's sad to say, but this is pretty much the only use I have of GNC. Their staff is typically friendly, have knowledge on select products, and never trying to be pushy sales-wise. Personally, I'd rather take my chances with some of the locally owned supplement shops.