Some (not all) of the services listed seem like they could be very useful but I am verging on the sceptical side. Perhaps clarifying these points will ease my doubt:
1) Who is behind this? I can't find this information easily on the website.
2) How and where from are the testers acquired?
3) Do I receive any demographic data about each tester? I.e.: location, age, etc...
4) Has anyone here tried using any of these services?