I have the original EOS 5D, which has a VERY strongly center-weighted AF system, with a diamond-shaped array of 9 AF points, all clustered VERY tightly in the middle: this same,exact AF pattern and size was lifted virtually intact from the EOS 30D and plopped into the 5D. This means that the AF system is very centrally-weighted, and when doing focus-and-recompose close-range people work, the AF system is not as reliable as a camera that has a wider spread of AF points that cover a larger spread across the finder.
Canon lovers hate to hear anybody dismiss the features or quality of Canon cameras, but I own a reasonably complete 3-body Canon system and a very wide,complete Nikon system; there is NO comparison on off-center AF performance between a newer, better Nikon and a 5D-series camera. In indoor,lower-light situations, the 5D's extremely center-weighted AF pattern leaves the entire periphery outside of the centrally-located area devoid of focusing points; this is one of the main differences between the 5D series and the 1.6x bodies, which have the SAME-sized AF diamond, but the AF covers a wider area, since the frame of 1.6x is smaller. No offense to the 5D series, but it is built largely around a $350 EOS Elan-level body, with a very capable sensor and image processing system, but the "camera" part of the 5D series is quite weak and primitive in comparison to older film bodies. Even the new 7D is built upon a much more-capable body and a higher-performance AF system than the 5D series.
If you want really killer autofocusing, you need to look someplace else besides the 5D series bodies. Everything is "relative",and with high-speeed professional lenses of long focal length, like a 300/2.8 or a 70-200 at 200mm and f/2.8, the 5D can autofocus "pretty well". However, there are other cameras that can focus "superbly" with those same lenses, AND with lesser lenses. I'll come right out and say it: look at an EOS 1D-series body, or a Nikon D700 body if you want to see a an AF system that is at "the next level". People love the 5D and 5D-II, but it's like a Honda vs Porsche comparison, or a Shakespeare vs G. Loomis, or a MacDonalds vs Bertoli's Family Italian Restaurant with Papa Guido at the stove.
You pay your money and you make your choice. There *is* no comparison between a 1Ds series body and a 5D series body. One is basically a $350 body and the other is a flagship-level camera engineered to sell at a $4800 price point.