The Anandtech review says it gets 85% color gamut after calibration, meanwhile the Dell 27 inch ultrasharp gets about 95% color gamut. The issue with the Apple is that it uses white LEDs instead of using RGB ones. It limits the color space slightly. The Dell uses CCFL instead of LED, which means it results in better color gamut.
On the other hand the Apple really wins with the LED and glossy because you end up with better contrast and brightness. So it depends on what you need. Personally 85% is pretty good in my mind, but like I said it really depends on what you need.
Check out the Anandtech review, it goes over the color accuracy pretty well.
http://www.anandtech.com/show/3946/apple-27inch-led-cinema-display-review