The 5D Mark II is a great camera, and I have really enjoyed using it so far. The low-light capabilities are excellent. Even shots taken at 6400 ISO are not excessively noisy, and shots taken at 800 ISO look essentially perfect. That is useful for shooting at smaller apertures in situations without all that much light.
One very minor flaw, which is nonetheless annoying, is that the indicator for the number of shots left before your memory card is full tops out at 999. Even at full resolution, my 8GB compact flash card can store well over 1,000. People with 16GB or 32GB cards must be even more annoyed. You can argue that you only need to start worrying about how many shots you have left once that number falls between 1,000, but it still strikes me as bad design. For instance, if you were on a multi-month expedition somewhere far from computers, it would be useful to know how much space you have left each day.
If they are hell-bent on keeping the display to three characters, I would recommend expressing it as a percentage. Seeing ‘80%’ or ‘60%’ is a lot more useful than having ‘999’ flashing at you all the time.
Mark III designers, are you listening? If so, I would be very happy to serve as a tester for any prototypes…