Just a week ago, my dry cabinet has 5D1 and 5D2 in it. I just sold the 5D1 and topped up for the 1DsMark2. I can't say what you will miss for 1D Mk 2 but here are some of my comparison with 5D1:
1) ISO performance for 1DsMk2 at 100% pixel peeping is probably 1 stop worse off than 5D1. At 5D1, I don't mind using ISO 3200, but 1DsMk 2 ISO 3200? I will avoid it. Please take into account that 1DsMk2 has more megapixels and I didn't try upresizing the 5D1 images.
(please don't flame me for this; it's just a personal opinion).
2) 1DsMk2 auto focus is really much quicker than 5D1.
3) 1DsMk2 battery seems to last forever compared to 5D1.
4) 1DsMk2 shutter lag is really much better than 5D1
5) 1DsMk2 controls are harder to use - but still get used to it within 1 day of usage.
6) 1DsMk2 4fps feels much faster (maybe because of better focus and shutter lag as well).
7) 1DsMk2 is heavier but for most guys in normal usage, it's not too bad. And you'll appreciate the good build.
Still I have no regrets, overall I felt that 1DsMk2 at the 2nd hand price was a bargain and a worthy upgrade from 5D1.