Honestly, I never seriously considered the 5DII. I did, however, consider the 5Dc since it seemed to me to have slightly better IQ for a fraction of the cost. Both however, especially the 5Dc, are showing their age. It didn't make sense to me to spend as much on a 5DII as I did on my 7D for a body that was older, and had a fraction of the features for moderate IQ improvement. Good glass took priority over that.
The 5Dc was more appealing than the 5DII for it's low cost, and, arguably, better IQ. For most photos, the difference in IQ between the 5Dc, 5DII, and 6D (and other Canon full frames) won't be noticeable. When pushed however, the differences are apparent. And I appreciate the ability to push the ISO when needed for those situations when flashes either wouldn't work well or are just plain out of the question. Not only is there less noise, but they've practically done away with banding in the shadows. And being able to focus better without the AF assist light is also good for when you're trying to not be obtrusive.
Quoting from one of those posts:
"I just meant that the 6D has a lot of 'bells & whistles' that the 5D does not have.
Better chimping screen.
Built-in to the menus flash control.
DOF preview on the right side of the body.
Higher availabe ISO.
In-camera processing options.
To me, the main things there are sensor cleaning, live view (for the odd occasion where it will be useful), and better battery (which I already have from my 7D). Honestly, though the WiFi may seem gimicky, I'm sure I'll be able to find a few good uses for that kind of live-view remote control of the camera. One point not mentioned there is their improved silent shutter, which actually sounds like a decent noise reduction compared previous camera generations (7D).
I can justify the price difference between the 6D and the 5Dc based on these advantages. I can't justify the price of a 5DIII (or the 5DII for the few advantages it would provide), given that I will be hanging onto my 7D and not trying to be some pro photographer. Photography is just something I enjoy spending my time on, amongst other expensive hobbies that I still have to find a way to pay for.
I'd say many others fall into a similar category as myself. That and many consumers don't want to go and buy a 5- or 8-year old camera when there are newer technologies available which will be supported further into the future. It's really just going to be up to the buyer, informed or not.
Reading over all this, damn. I'm sounding like a Canon rep or something. I guess I'm just trying to point out that it's actually a worthy camera, which to be honest I had my doubts about at first too when it was announced. Then I did some looking around and was happily surprised by what it actually had to offer.