I'm a member of a couple, and as above they don't generally pay a lot and it can be annoying when you get a good one but you don't qualify (ie it might be targeting pregnant people etc etc).
Some sites work on a points basis, but it takes ages to build up enough points to cash them out. The better ones pay like mentioned above, but it still takes a while.
I've gotten a few $20 gift cards so far through one.
Also through the surveys themselves a sample pack of washing powder once and a sample box of cereal another time. And like a $10 coles voucher for spending half an hour on the phone in a survey about nappies or something (yes I had a kid in nappies at the time).
But the really good ones like that or ones that do pay a bit more than usual are not as common as you'd like, well like $2.50 ones are relatively common but you don't always qualify to take the survey.
I think in the US there are probably ones that pay better or have cooler rewards (like product testing etc).
I personally stay away from ones where I'd have to buy stuff I don't normally buy and I don't like to give my mobile number for text messages etc.
