I come at this from a Canadian coin perspective.
I like heritage, good deals can be found, but you have to be patient. I think the key is to remember to add in all costs and set yourself a bid limit. Buyer fees become irrelevant at that point. Great resource for past sales and pictures. Customer service has always been great and responsive for me.
Purchased a few things on Stacks, happy with purchase and quality, but not many great deals.
Greatcollections, found some great deals there, but very sporadic, only had a few wins there, lots of items seem to have really high starting bid prices.
icollector, some real gems, but lots of questionable auction houses, really read the fine print before buying.