I have a 1DS III at work that I'd kind of forgotten about. I was typically opting to use a 5DII, and then the III when it came out. However after shooting with the Mark III a bunch, I've been really liking the lower ISO image quality that comes out of the older 1DS. Not only that, I really do like the feel of a 1D style body, it's just nice! The only thing I miss while using the 1DS is the high ISO performance (that even the 5D2 beats), and the newer autofocus system. That being said, I only miss these attributes in very specific situations that I don't deal with that often.