I JUST downgraded. I had a 5DII, 5D classic that I have had for a long time, and my 20D. Those monstrous 21 MP files crashed my computer every time I opened a RAW image. I couldn't edit one image without my entire system crashing. I did love the video, and I did love the sensor cleaning. I also loved that beautiful LCD screen - it was to die for.
However, in terms of IQ, I am more comfortable with my 5D classic. I don't know why that is. I felt like the 5DII was more vivid in color - and I also felt like the midtones were brighter (on the 5DII). Having shot w/ the 5D classic for so long, I felt more comfortable editing 5D classic files. I felt like I was fighting the vivid color from the 5DII. This isn't saying that the 5DII is a bad camera, I am just saying I struggled editing to my liking with it. I hadn't gotten comfortable with it yet.
I did some ISO comparisons and couldn't really tell a huge difference between the 2 cameras.
Examples when I got my 5DII in May 2009:


Anyway, the 5DII is a fine camera, sharp as can be. So is the 5D classic. I love my 5D classic and will never sell it.
These were my very first pics with the 5DII right out of the box that first day and a few days later:
But, I did end up selling it just last week. I downgraded to two 5D classics and pocketed $1100 difference. For now, I am happy with that.

Best of luck with your decision.