Im by no means an expert, but being that my wife is a photographer (children, newborn, families, maternity, engagement... basically anything but weddings) and recently picked up a 5DmIII to add to her toolbox here are my (secondhand) thoughts. She has been using the 5DmII for some time (got one new when they were released) and also has a 1DmIII. She LOVES the new 5DmIII. The autofocus is amazing, the low light abilities has allowed her to use her flash less (which is a blessing when working with children) and the RAW image quality is the best of any of her cameras. The new processor allows her to shoot high speed (again a blessing with children) and never have to wait for the camera to write to the card. While 6 fps isn't fast for the sports world (or the dolphin world

) it is plenty fast for portraiture photography. I know a lot of people use the 5DmII (and now the III) for video, I can honestly say that I don't think my wife or I have ever even switched one of the 5D's into video mode. We have a Canon VIXIA HF G10 for that... Comparing the 5DmII and 5DmIII side by side, I can tell a difference. Mainly in the colors, sharpness of background focus, and the awesome lowlight abilities. Again, im not an expert (just an underpaid assistant) but for someone using it for photography, it a great new camera.
As a side note, my wife prefers the 5D body over the 1D body as she has small hands. Now that she has her 5DmIII, I think the 1DmIII is going to sit around in the bottom of her gear bag for a while.
If she could swing the extra $3000 for the 1Dx (and find one for that matter) I think she would have gone the route more then anything for the dual CF card slots. The "dual" card slots on the 5DmIII are pointless as she hasn't used a SD card in forever.