It costs something, I forget how much. It lasts for a year before needing to be renewed. It gives a discount, I forget what rate, on almost all purchases.
A couple of years ago I needed to buy just over $100's worth. It was worth joining in order to get the discount on just that one purchase. I haven't needed to buy much there since so I let my card lapse.
It would definitely suit the serious buyer of things that only Hobbyco sells. Once you have the card you can also get the occasional toy or gift that you would otherwise have bought at Kmart without paying the Hobbyco premium. So if you work in town Hobbyco could easily become your one-stop money sink.