I took a few random tests as a user on all three of the sites listed. The interface was fine, I was motivated to try a few more tests and it seemed as though it would give good feedback to designers, provided they had asked the right questions and kept their tests simple.
It was occasionally slow to the point of hanging when requesting a new test, the number of tests was not as high as I would have expected, and on occasion I was given a test I’d seen before.
There are paid options, but users can get access to testing services by acquiring ‘karma points’, obtained from testing other sites
You learn as much by looking at other people’s tests as you do by testing your own material, in my opinion