So far, yeah... it's junk.
The new TestFlight app can only be installed on devices with iOS 8 or higher.
The initial review took 24+ hours and they rejected us the first time on a non-existent issue. I appealed and they said it was their error, but they made me submit again anyway. Subsequent build reviews don't seem to actually happen so it does become available immediately.
The new TestFlight app installs the app, but the progress bar never completes.
It doesn't actually tell you what build your testers are running or if they've even installed, so that's worthless.
All-in-all, the only advantages are that I don't have to make a new AdHoc certificate when new beta testers come along, I don't need to collect device IDs, and we can have up to 1,000 testers. Not sure if those benefits are quite worth the issues yet.
So.... 2 out of 5 stars.