I think they are pretty good. I've joined a few and have purchased off 3 of them. The main things you have to be wary of is knowing the value of the thing being sold, and buying stuff just cause it is a good deal. You can end up dropping a lot of cash, because you feel the deal is too good. Buy things that you were going to buy or do buy regularly.
As for the value, the sites sometimes make it sound like you are making a killing, when you actually aren't. Helps to do some research if you are unsure.
I've gotten some great deals though. Took my family to Luna Park very cheap, had some great meals, bought perfume (gifts!). Stuff like that