I had a 1Ds for a while. Lovely image quality, good size files, able to take plenty of my heavy-handed PP, super build quality, reliable AF. Basically, every shot out of the camera was high quality.
The problem for me was that the write time was very leisurely and I found I was missing shots of my kids as the camera was just a tad too slow. Probably not a problem for most, but I tend to shoot in bursts.
In the end I switched to a 5D (and then to a 5DmkII). The original 5D was just a bit faster to operate, and to be honest I like the smaller size. IQ was very close to the 1Ds (if pushed I favour the 1Ds, just). Hi ISO on the 5D was also a teeny bit better.
The 1DmkII and 1Ds share batteries/chargers don't they?