I have been shooting with a 40D, and just added a 5Dc to the mix. I haven't had a real good chance to try it out yet, just a few tests inside, but my conclusion so far is...there's not much difference. Noise straight out of camera is noticeably different, but not by much. 10 seconds in LR and you can't tell much difference. There is a bigger gap in color noise, but color noise is easier to get rid of.
I never thought of my 40D as all that "fast" before, but after going back to it after the 5D, it is like lightning!
The main reason I added the 5D was because I needed a backup camera for weddings this summer, and I wanted to try out FF. At this point, I am not convinced it was worth the extra money over just getting another used 40D which I can get for about $300 (vs. $600 for the 5D).
The biggest noticeable difference is the shallower DOF of the 5D. Shooting inside, with the 50 1.8 on my 40D, and the 85 1.8 on the 5D, there is a very noticable difference in DOF while having a similar FOV.
I will not make a decision on the 5D until I use it to shoot the 2 weddings this summer...but it may be sold in favor of a used 7D.