I got this one second-hand after the 5D. Had to sell the 5D to finance it. Anyway, no regrets (although my occasionally aching right arm would perhaps disagree).
Noise was a very important issue for me (I often shoot a whole evening at ISO 1600, since I hate flash), and coming from the 5D I'm picky about this. So is the 5D or 1Ds mk2 better in the noise department? I would say that both are about equal. The 1Ds mk2 does have a bit more noise at 1600 and 3200, but on the other hand its noise seems to be a bit more even than the 5D's (due to better electronics design and electromagnetic shielding, I would guess) and thus less annoying.
At first I disliked the loud shutter sound (5D is much softer-sounding), but now I'm totally used to it and even like it a bit!
I have not so far noticed any significant difference between the 5D's and 1Ds mk2's autofocus. Some people say that there is a big difference but I have not noticed it. In whichever case, it certainly isn't worse. Maybe if I used servo AF (which I rarely do) I might notice a difference.