It's been a real education to read some of the replies posted in response to this question, such that I feel compelled to add my own 2 cents' worth.
I upgraded from an EOS40D to the 7D in 2010, partly because of the reportedly much better quality sensor in the 7, partly because of the increase from 6.5 to 8.4fps [I shoot a lot of hawks, owls, buzzards]. I carried a set of L Series glass with me to the new camera: 16-35mm f2.8, 24-70mm (Mk I) f2.8, 70-200mm (Mk I) f2.8, 100-400mm f4.5-5.6.
In almost all respects I find the 7D auto-focus to be a step back from the 40D. I think there are several reasons for this:-
1. You have to partner it with the right lens. Shooting raptors with my 70-200 I get 2-4 times as many pin-sharp "keepers" than with the 100-400mm. Yesterday I shot 650 exposures of the Olympic Sailing in Weymouth with the 100-400mm and less than 1% are keepers. In *all* cases it's [poor] focus that has spoiled the shot. Note that the 70-200 has USM, the 100-400 does not: that makes a *big* difference.
2. I think [I've read but now cannot find] that the 7D actually uses a different, "newer" AF when compared with the 40D. Perhaps this is the equivalent of a "Microsoft Version 1.0 Release"? [ i.e. they shipped it in order to get poor unsuspecting users to help identify the bugs].
3. This might be complete nonsense, but in my highly subjective view the worst possible thing you can give this AF system to work with is a tree-line in sunlight. The broken "surface" of the trees coupled with the hot-and-cold spots caused by sunlight and shade are enough for the "You gotta be kidding me!" warning light to appear in the HUD (if there was one, which there isn't). In other words, perhaps my experience is pushing the camera/lens combination to the limit.
4. I wonder - and I don't know - if the 7D is particularly fickle to "micro-adjustment" requirements for Canon lenses. I have not tried this, and I have no way of knowing if it's good, bad or indifferent, but last night, after a poor result from the Olympic shoot, I went in search of enlightenment and found this:
As the article explains that there is a "reset" option to keep everything safe, I am tempted to give it a try...
Bottom line, I find that when the 7D gets it right, it is capable of breathtaking shots, head and shoulders above my 40D. Unfortunately, my 40D returned 5-6 times as many "keepers" as the 7, and in *all* cases the reason for ditching 7D exposures is focus.
By way of comparison, I recently decided to treat myself to a 'go-anywhere' camera upgrade. I'd been using a Canon Powershot G12 since they came out [G9 before that, Ixus before that], but on the recommendation of a semi-professional photographer friend of mine, was lured to the Panasonic, and bought a Micro-Four-Thirds DMC-GX1 mirror-less. Wow. It doesn't have the frame rate of the 7D, but *every* shot is pin sharp. Even the kit lens turns in very respectable images... and put a decent bit of glass on it [I recently purchased the 7-14mm ultra-wide] and it just blows my 7D away...
I'm so hacked off with my 7D that I want to sell it. Would love a 1DX but need to wait for the price to come down and the savings to build up to match it... I asked my Father [who inherited my 40D, if he'd swap it for my 7. He knows the trouble I've had with this thing and the rotter won't bite...]
How about a 60D with under 4k clicks? Ill bite on the 7d