frankyscouse: some of them are a con
Yep. I've had my best luck with Daily and Clearvoice. True, they don't pay as much, but just getting paid is good enough for me. Lots of people get ink, something they'd buy anyway. I don't, but I've heard that the ink is of good quality.
If you're willing to put up with spam for a while, Therapedic and Instaprint offer free credits for no money.
If you dine out, you can get creds from Restaurants.com and save money.
The biggest thing is to not buy something you don't need. Better to just buy the credits.