Mine has just been delivered (seems like most people are getting them today!)
Agree with what's been said already that the screen really is stunning. You really do ignore the bezels and the design once your using it, the colours and clarity are fantastic.
I was one of the folk worried about the performance with the 13 inch, and it's not as good as I'd like it to be. Mission control lags, resizing certain app windows causes a lot of stutter and 4k videos full in Chrome (no option in safari oddly) do lag and stutter if you do anything else on the machine at the same time.
I've plugged a 15 inch into it (with the 455 card) and performance is smooth as butter. Mission control and animations experience no lag, 4k video works full screen whilst I'm doing other bits on the laptop screen, with no stutter or reductions.
The 13 inch doesn't do a terrible job, but it's not as smooth as I'd like It to be. Looking at iStatMenu, the CPU is being absolutely hammered when playing 4k video in Chrome, so this could be something that get's better with software and performance updates.
Only other observations as pointed out already, are that the MacBook (15") does get quite warm towards the top of the touch bar with the monitor plugged in, and also the monitor isn't very sturdy. I have a fairly crap and unsteady desk, but the monitor is wiggling a little as I type with my elbows rested on the desk. YMMV.
Going to have a play with it for a week or so, and see what I think.