To expand on my earlier remarks. I just finished by regulary scheduled profiling of my early 2010 manufactured 27" white LED backlit iMac using the new update of Xrite's i1 Profiler software with the i1 Display Pro colorimeter.
As you can see from this composite (fig.1) comparing the iMac's display to Adobe RGB(1998) and sRGB where the iMac is weakest is in the deep blues and cyan but over all is better than sRGB. This composite was made from screen shots of ColorThink 2.3.
This is confirmed by i1 Profiler where one of the green patches of Xrite's ideal values for a 24 Patch ColorChecker is slightly in the "yellow" zone and one of the blue patches is slightly into the "red zone". (Fig. 2)
There is also some slight fall off in illumination in the corners of 27" iMac's screen (Fig.3 also from i1 Profiler)
However overall the display has been very stable over the past 50 months I've been using it as you can see from the trending report (fig.4)
My real world experience is that the iMac is fine for general portrait, commercial, and landscape work. If you were working with very specific brand colors like Pepsi Blue or Coca-Cola Red, or for high end fashion work you will be better off with a new Eizo CG or a high end NEC.
My other display is an Eizo CG222W with CCFL backlighting from 2010 that had the screen and backlight assembly replaced (under warranty) by Eizo in 2012 and has about 1900 hours on it since the rebuild.