I have a 5D mark 2 and I have never ever once wished for more megapixels. In most cases, the quality of the optics limits the total resolution more than the pixel density.
The weak point of the 5D2 is the AF. That seems to have improved a lot in the 5D3.
I am extremely happy with the full frame sensor. Often for tele pictures like birding I use it as a 200% viewfinder for an APS-C-sized crop. For wide to medium focal lengths I rarely crop. The 24-105 is a great match for the camera.