I went for it in the end, after watching a few forums for any problems. SIM-free, so can't comment specifically on T-Mobile.
First impression was a noticeable "smoothness" feel. I use Nova Launcher Prime, and it seemed a little more silky smooth straight off. Almost as if the animations had slowed, but the frames-per-second had increased. This is also true when using the phone at a "never will I really use it this fast" pace - the smoothness keeps up. Huge improvement on panning around webpages too, especially when they're still loading. No more lag! Noticeably faster unlock transition from lock screen to homescreen too.
Second on the list was Google Now for me, set that up and have had a good play with Google Voice. It's pretty decent. Not quite got the integration I'd hoped for yet, but perhaps that'll improve (he says, hopefully). An example is sending a text. You can say "text Rob, I will be there in ten". All it does is find Rob's details and offers me a link to open Messaging where I type a message. Bit disappointing. Same with alarms - doesn't understand tomorrow or dates. It does, however, work with facts and other bits of functionality too so it's not all bad. "Wake me up at 9" does in-fact wake you up at 9.
I had no problems other than the usual fight with Kies...pff.
Edit: Oh, and expandable notifications are pretty neat.
