Just been fooling around with the loaned camera- boy it takes nice pictures! The 5DmIII body is a joy to use- it is pretty much a combination of my 6D and my 7DmII, but it's more than that too, the feel in your hand and the button placement is very natural (it's a slightly bigger body than the other two, and I have big-ish hands), the menus are very intuitive, and the action of the shutter is gorgeous!
The lens is fantastic- but boy is it heavy! the fact that this is an f/2.8 all the way through from 70-200mm, combined with about 4 stops of IS is a winning combination with a sharp lens like this, but it's surprisingly not that much better than my f/4 70-200 non-IS. Once Christmas is out of the way it'll be fun to try both lenses on my 6D and the 5DmIII and see what I like best :)
But though I love both my camera bodies, and it's a huge advantage to have both as I get the best of both worlds and have a second camera both to make shooting in a hurry easier and as a backup, I have a horrible suspicion that trying this 5DmIII may have been a huge mistake as I'm already feeling terrible angst about having to give it back in a weeks time ;)