I have the 27" Apple Thunderbolt display in my office that I use with my Macbook Air. It's the same display as is used in the 27" iMac, and I love the display.
At my home office, I have an Eizo ColorEdge CE240W attached to my MacPro which I use as my photography workstation. After enjoying the Eizo display for about 5 years, it's hard to give it up for a shiny new 27" Apple display. The color calibration and gamut is superior on the Eizo. Plus, I use 2 external eSata towers for photo and video storage and back ups.
The iMac is beautiful, but I think that the display, as nice as it is, is not up to the task of critical color work.
Cheers.
Bud