I joined UserTesting a couple of weeks ago, haven't been on it much but it's pretty much the same as WUD, most tasks have screener questions and after you complete your first paid test they review it which takes a few days. They give you feedback and then you can continue testing. They pay $10 per test in to Paypal which I think worked out at £7.52 if I remember correctly. If you leave the webpage open it dings whenever a new test pops up.