I've owned the 1Ds, 5D, 1Ds2 and now 1Ds3. Probably 50,000+ clicks on those bodies across a 4 year period. A couple comments -
The 1Ds2 resolve more detail than the 5D; the 1Ds2 also has significantly more detail dynamic range than a 5D. If a smaller camera is important to you, then 5D makes sense, but if the goal is the best sensor / image quality - the 1Ds2 easily wins this comparison.
The 1Ds3 compared to the 1Ds2 is disappointing IMO. The 1Ds3 has slightly less dynamic range and is more likely to clip clouds and skies. The ISO 100 shadow noise is about the same as the 1Ds2. The 1Ds2 raw files appear sharper. When a 1Ds3 raw is downsized to the same size as a 1Ds2 raw, the files are very similar. So the good news is - the 1Ds3 isn't capturing less, but I don't think the 5 extra MP or capturing anything more either.
The 1Ds3 benefits are mostly the features - larger viewfinder which is huge, larger LCD, good high ISO (compared to the 1Ds2 and 5D), live view (very handy with manual focus lenses), a bit lighter, simpler user interface (though the AF point selection is utterly lame).
If I had to do it again, I may have stayed with the 1Ds2. I also had a Phase One P25 back and its files are categorically better in terms sharpness and dynamic range. Had Canon used a weaker a AA filter on the 1Ds3, then I feel it could have really challenged medium format. Instead the AA filter (or whatever is softening the raw files) zapped alot of the 1Ds3's potential.
All of this is very relative - it depends on what camera you had before, what you expect a good raw file to look like, etc. Coming from the P25 back, I'm not impressed. I love the ergonomics and handling, but sensor / image quality comes up short.