I have owned a Canon A1, 20D and 1D MarkII N.
I mainly shoot sports events that sometimes have poor lighting conditions. To take razor sharp shots with correct color in these conditions, I use 'L' primes with wide open f-stop (1.2, 1.4, 1.8 maybe 2.0), and a fast shutter speed.
The 1DM2N has a fast frame rate and solid auto focusing systems. The battery life is excellent.
I've compared the 20D to the 1DM2N and have noticed the 1DM2N sometimes auto focuses faster then the 20D using the same lens in the same conditions. Most of the time, I notice no difference in auto focusing speed between the 1DM2N and 20D.
The AWB in the 1DM2N is poor when compared to the 20D. Sure, the AWB can be adjusted manually but what a pain compared to the 20D AWB that gets it right most of the time. Perhaps, the technology used in the 1DM2N is older and maybe starting to show its age.
The user interface on the 1DM2N is difficult to use (hold down two buttons with the left hand when turning a knob with the right hand, release a button to select) it hurts just thinking about it. The 20D user interface is a much improved UI designs, faster to use and much less finger movement. The Nikon D70, D200 UI designs are better yet.
The 1DM2N battery lock down and memory door design is poor compared to the newer 20D with a grip.
The 1DM2N camera is heavy, costly and a lower value when compared to newer models. Taking pictures with the same canon lens, in the same conditions, the 20D can deliver the same image quality (correct color and sharpness).
The 1DM2N can take more pictures in the same amount of time but make sure you adjust the AWB for the lighting or they won't have correct color or sharpness.
I for one would like Canon to improve the 5D or 30D with a better low light, Auto Focus system and faster frame rate.