Nick Arnott has taken a look at TestFlight a year after its relaunch and I agree with his overall take. I write about apps for a living (I currently have 74 betas in my TestFlight), and the simplicity of Apple's system is unparalleled. I just need to give developers my email and that's it.
I also agree with Nick's comments on frustrations with using TestFlight. I hope that Apple will continue tweaking these aspects going forward.
I've experienced a few minor frustrations with as a user though. For example, I can't accept an invitation from my computer — I have to accept an invitation from the device I want to test on. Also, TestFlight emails don't contain any release notes. With other services like HockeyApp, developer release notes are included in the email, so you can decide from the email if you care about the update or not. Lack of these release notes from TestFlight means you'll have to tap through to the app and view on your testing device to see if you want the update or not.