Philo has just put up a really well-done comparison of two of the color sensor that are currently available for the NXT. The first is the official one from LEGO, included in the V2 NXT kit, while the second is the HiTechnic V2 color sensor which can be purchased separately. I'm not going to bother summarizing Philo's excellent presentation too much, because reading it is an education in itself - not just on how the sensors "perform", but on how to compare sensors properly: developing tests, and understanding the results. Please read his page for the whole scoop. A dirt-simple summary is both have strong points, with the HiTechnic one being amazing at detecting multiple colors and handling challenging ambient light conditions, and the LEGO one (by virtue of using the rapidly-reading AtoD converter on-board the NXT) being much faster (probably important for line-following, for instance).
Thank you Philo for doing comparisons like this - and please read his summary, it really is excellent.