I am strongly considering it, as the Mac Pro update didn't happen, and this machine will outpace the 4-core Mac Pro.
Points I will look at:
1. Color quality of the screen, and how well it will calibrate. As it is an IPS display, I expect it to be great. I hope for natural colors, not for oversaturated colors. I will also check the coating of the screen, which is rumored to be 75% less reflective than the glossy displays (how much more reflective than the matte display will it be?)
2. What displays are available to connect, and do I need dongles? (I would neer use a laptop screen as a main editing screen.
3. Will I be comfortable to use the screen as a second screen, e.g. for tools?
In short, first I have to see one, and then I'd like to read reviews of photographers on this machine.
I have no empirical statistics to help you, but I went down to my local Apple Store today to check it out. Colors looked great and the screen seemed to have great contrast. No idea on calibration though.
I think the two Thunderbolt ports plus HDMI port would have you covered for most displays (of course, you would need an adapter for the Thunderbolt ports more often than not).
And of course your last question is a personal question and impossible to answer. What I can say is that the 1920 x 1200 display "resolution" (as in, the "most screen space" option under Display settings) made text quite small but there seemed to be a lot of room on the screen in Aperture for a photo.
I don't know if this helps at all, but figured I'd try.