I got my UA a few days ago. Love it. I wish I still had the PA to compare it to, but it seems that the UA is a little easier to dial in some smooth dirt.
I'm using both a Finally and the UA as add-ons to my Helix Stomp XL. I love the Helix, but I also love the ability to change my tone with minimal fuss. And of course anyone that knows me knows that I won't shut up about how much I love the Fender tone stack
Direct comparison UA to the Fatdrive - the FatDrive is still the Fattest. I seriously haven't heard a pedal preamp that compares to the sheer bootiness of the FD. The UA is the closest I've seen so far though. One thing that the UA does better is dirt. The FatDrive does the Fat part really well, but not so much the Drive part (at least as far as getting some dirt). It could be the tube, but the UA is easier to dial in some smooth grit - just the right amount.