I received my u2913wm today.
It is a bit strange the first time you see this screen for real
My first feedback is that it is a decent gaming monitor for a screen not designed for gaming in the first place: reactivity is good and I have not noticed an increase in input lag compared to my monitors. I do not have tools to measure those two data but in terms of feeling, it is OK.
For the games I tested, I saw that some games like Borderlands 2 and Assassin Creed Revelations manage the 21/9 format as if it was triple-monitor: the hud is positionned in a virtual central 16/9 screen.
I tested various games. Assassin creed 3 does not manage 21/9 at all. It just sends a 1920*1080 image (with black bars on the sides) even when the correct resolution is selected.
This format is great for gaming. When games are supported correctly, I do not really see any good reason that could make one prefer 16/9 over 21/9.
I do not think that this format can replace a 3*1 set-up. It is just not wide enough. It would require something between 24/9 and 32/9 (corresponding to 1.5 to 2 16/9 screens wide) to really compete with triple wide.