I recently upgraded from a T1i, and I had a terrible time deciding what to upgrade to. I bought and returned a 7D. I think the 7D AF system is overly complicated for what it provides, but I am also not the 7D target market- I am a "mom with a camera". I also thought the image quality was no better than my Rebel, and that it amounted to a big heavy camera that was still a crop sensor, and frankly, I just didn't like the camera. So I didn't know what to do, and I rented a D7000. The camera had amazing, very fast, AF. Grabbed and locked very quickly, worked well. But, I could not justify selling all my lenses, flash etc. to switch to another crop sensor camera. And most importantly, I do not prefer Nikon colors, or the contrastiness (not a word, I know) of Nikon files. You can address it through editing, but it seemed like it would be a constant struggle.
So I bought a 5DMII, and based on everything I has read about the focus system, I expected pretty much all of my photos to be out of focus. I thought it was going to be just horrible. And I have found that via One Shot or AI Servo, shooting my toddlers, my keeper rate is very high. I do find it hunts in low light. I do not use the center point exclusively, I do toggle, and I find the outer points are ok in most situations. No, I probably would not choose it for a serious birds in flight, sports camera. It doesn't necessarily feel like a "fast" camera. But overall, factoring in the size and weight, and the image quality, and the affordability of the camera relative to other full frame options, it is the right camera for me. A lot of extra information there, but to address the original post, I don't think the AF is "that bad"- other cameras have better AF for sure, but I think that people make it out to be more of an issue than it is. And I decided I would rather stay invested in Canon and hope that by the time I upgrade, the AF will be better. But the AF is good enough for most situations I photograph.