Very briefly, since this isn't necessarily objective nor scientific... but I feel that the Mark III is orders of magnitude better than the Mark II.
I bought a Mark II at the beginning of the month wanting to save money and/or spend the difference in price on better glass. It's the same dilemma I see posted about time and time again.
Autofocus: I'm a portrait photographer (children and newborns, primarily) who is getting back into weddings, and I have to say that the new autofocus makes a WORLD of difference in the ratio of usable photos. In editing after a simple demo session with my own son, I only eliminated a handful of photos due to focus misfire, as opposed to the 1/3 I usually do with the Mark II. This will make a huge difference in saleable prints. The same goes for the noise level in higher ISOs. ISO 3200 looks like ISO 200 on my old 30D (likely much better).
Metering, Sensor, & IQ: I don't know what it is, but even though I shoot manual, the contrast and color quality of my shots came out so perfect that I could get away with no post processing in Lightroom if I was feeling lazy. That's a huge time savings in workflow. This makes a big difference when sorting through 3-400 images per shoot.
Mind you, I just upgraded from a 30D to a 5D Mark II just this month, and that upgrade didn't make me as giddy as the move from the Mark II to the III. And this is after only 20 minutes of shooting. I'm sure I'll have more opinions in the next few days.