Have you used it? How would you rate it?

I rate it 10 of 10. It's pretty awesome, and the Testflight SDK has some additional functionality that's pretty useful (such as allowing testers to write comments in app).

I've used it before, and I would consider it as close to a seamless experience as you can get. Though you are limited by Apple's ad hoc limit of 100 users.

I've used it. Works fairly well, of course your friends need to give you their device ids, and you'll make an ad hoc build.

Hasn't been updated in a while, and has some serious bugs and usability problems in use in a real testing environment with more than 50 or so users.