I have been performing website usability testing for several companies since November. I have done the most tests for User Testing (over 50) and Try My UI (about 10). Both sites are similarly run in that you get paid $10 per website test.
User Testing has been a way to consistently make some extra money while working at home. Besides the usual testing, they do surveys to do more focused testing, which can earn you significantly more money for not much more time. In early December I participated on a panel, where I gave 20 minutes' worth of feedback for seven days and earned $150. User Testing consistently pays seven days after testing and there have been zero issues. I also rarely encounter issues with their testing software. Testers are generally screened prior to being approved to do a particular test and it is really hit-or-miss as to whether I am selected. That being said, I have earned nearly $700 over a two month period. I recommend this company. They do exactly what they state and have been very professional every time I have spoken with or had a discussion via email.
TryMyUI.com has been much more inconsistent. I have had a number of issues with accepting tests only to be told another tester completed the test after I had started the test. I have also had issues with websites not functioning that have taken a number of tries and phone calls to TryMyUI.com to resolve (at the risk of receiving a bad user review and ranking. By the way, their website states "talk to a live person" and provides a phone number. To date, I have telephoned them four times during normal working hours and got Google voice mail every single time. I have also followed up with an email. They currently owe me $70 and $50 of that is past due per their payment guidelines. Because of this and their non response, I cannot recommend this company. Should my issue of non-payment be corrected, I will update this review.
Update: After several emails and phone calls (none of which were responded to), I received payment for my outstanding tests. I would recommend TryMyUI with reservations. I do continue to test with them, but it is not the first site I go to when seeking work.