And having the 5Dc, the improvements in the 5D2 were not enough to make me change.
It's a tremendous difference for my use. For instance, the 5D was right at a resolution level that caused very ugly hair moire in 3/4-length portraits. The resolution of the 5D2 is great enough to fully resolve hair at even loosely framed full-length portraits. In fact, I can shoot a loosely composed group with the 5D2, enlarge it to 30x40, and count eyelashes.
Then there is Live View, which is even better than the beloved groundglass of my medium format cameras.
The 5D2 finally allowed me to fully retire my medium format cameras--which was a huge change of photographic lifestyle for me. For professional portrait photographers, the competition for the 5D had always been medium format, which is what most of us had been using before. The company the 5D has bludgeoned the hardest has been Mamiya. In fact, I'd guess it's the 5D that actually killed the Mamiya ZD, at least in the US.