We've covered TestFlight before, and I recently started using it to test a top secret app for AOL*. If you're a developer who wants to reduce the friction of testing beta apps for iOS, you absolutely must check it out. It makes beta distribution of your app a drop-dead simple process by leveraging a native TestFlight app and whatever beta you wish to deploy. Yesterday TechCrunch noted that version 2 of the service is rolling out, with even more goodies for developers.
What goodies? Many of the additions involve getting user feedback, a somewhat large hole in the otherwise awesome service. There are also hooks for crash reports and "check points" to monitor actual use in the app. TestFlight now also supports enterprise accounts so you can deploy your beta to as many devices as you need (provided you have an Enterprise developer account with Apple, of course).
While TestFlight is free for developers, they are reserving the right to start charging for some of these new features. I think that's a smart move. Things like check points are immensely valuable to developers who create complex apps, but less so if your app has a very basic feature set. It's also good for them to start monetizing this service so it persists -- as of right now it is absolutely a best of breed service that all iOS developers should consider.
*Feel free to guess what I'm testing in the comments. I'll never tell! Well, I'll have to eventually.