I've had the 5D MkII out in the dark against a D700, and 70 - 200mm f/2.8 sigma lenses on both of them. The Nikon refused to focus where the 5D MkII carried on albeit a bit slower than normal.
But...but...the D700 has over 5 times the number of AF points. So the Nikon AF has to be better, right??
I guess, the autofocus-feature on 5d mkII is rather a nice "giveaway" from canon, rather than really needed for the typical usage of the camera. It should just make it some more flexible. ;-)
- When using it for Movies (and this is probably the most obvious) you don't need an af.
- When using it for studiowork, you'll have all the time in the world to set a properly focus manually. As mkII tethered usage performs quite well, it's, in my opinion, the far superior method to focus manually than letting the af do the photographers job.
- When doing (wide) Landscape, there's in 9 out of 10 cases only one applicable setting to your focus-ring.
- When doing (narrower) Landscape and makro-stuff, manually focussing while in live-view and tripod-using has proven do be a good choise, regarding precision.
So for most cases, you would consider making use of a 5d mkII, it's probably the better option to do manual focus.
For shooting stuff like Wildlife the crop-cams are (due to the longer focal-length) far superior. 7d is, in this case also a better choise over 5d because of some few more fps...
For shooting sports it might either be the best choise to get a 1d-body or go with the 7d, as burst-rate is far superior compared to 5d mkII.
And in both cases - either 1d oder 7d - you get LOADS of more af-points of which you can actually make use of!
(don't take this whole post from me to serious!!!)