I went from the 5D classic to the 1DMKIII, the difference is thrilling. The image IQ is wonderful, especially for portraits, unsure what Michael is talking about but trust that he can see the difference. I personally believe the 1DMKIII is the finest camera I've ever worked with and I've been shooting many decades. I've also shot a 1DsMKIII and it's nice but nothing that makes me feel I "need" one, unless I really needed the extra detail.
The feel and way the MKIII works in my hands is so quick and easy, the AF with the 85L is stunning and I no longer worry over having that closer eye in crisp focus when shooting at 1.2. The microfocus adjust is very worthwhile as is the dust removal system but what seems most important is I no longer have to think about the camera very much, it's use has become intuitive and quick. Lately I've just left the 85L on it as my go to lens and toss a 24L or 2470L in the bag along with a 580EXII (but I hardly ever use onboard flash for other than a really low fill light).
The "look" I get out of the MKIII is most of what sold me on the camera, I contrary to Michael feel it has more pop and much better color than my 5D had and it was really good. On portraits, I much prefer the look I get out of the MKIII than the 5D. I have not used a 1DsMKII and was considering getting one instead of the MKIII and will still consider it if I really do need more detail as a second camera because they are so inexpensive right now compared to the 1DsMKIII but there is nothing that I've gotten out of the MKIII that tells me I need anything different.
That said, I'll be saving up for the eventual need to get a new camera body and anticipating that it will be a 1DMKIV when they come out, maybe. More than anything, I'm shooting available light, environmental portraits and the MKIII with the 85L is PERFECT! It translated to a 110.5mm equivalent focal length which is definitely a sweet spot for me when shooting portraits. On FF however, I prefer the 135L far over the 85L for portrait work. Both have excellent image quality but I just don't personally care for the 85L on FF for head shots and I know many will disagree with me and that's okay.
But if you do get the 1DsMKII, do consider getting the 135L, fab color/sharpness and bokeh.
I used to think I had to have FF until I got to borrow a MKIII for a couple of months. I became addicted to the 1.3 crop with the combination of lenses listed in my sig, everything is better for me with the 1.3 crop. I find the width of my lenses is enough in almost all cases, and find that curious but it works for me very well. If I truly need wider then I'll go get another (had one before but didn't use it) 16-35.