I wrote most of this when I was pondering an upgrade from my 20D body. I ended up getting a 5D Mark II. At the time, the 50D and 60D were both available and possible alternatives. The 60D was quickly crossed off my list, so my choice was between the 50D, the 7D, and the 5Dii.
The 7D came on the scene at just the point when (from my point of view) Canon took a turn with the 20D, 30D, 40D, 50D line and came out with the 60D (and later the 70D in the same spirit). My view is that this venerable line of cameras got "dumbed down" and the 7D was the answer for people like me who wanted a serious 1.6 crop camera. At this point I am unclear as to what distinguishes the 60D and 70D from the "rebel" cameras, but I have not followed the "rebel" line of cameras closely.
Some attractive things about the 7D:
It is clearly superior to (and in another league from) the 60D, so belaboring that comparison is pointless. It is, however interesting to compare it to the 5Dii. The big deal, of course, is that the 5D is a full frame camera, and you pay an extra $1000 for that.
A full frame camera does provide a wider angle field of view (and greater depth of field) from the same full-frame capable lens. There is more to it than that, the claim is that the larger pixel elements on the 5D full frame sensor provide improved image quality. Those big pixels certainly provide improved low light noise performance (which has never mattered all that much to me, given that I tend to always shoot at ISO 100).
Some people complain that the 7D will not allow ISO settings less than 100. All I can say is that if the noise performance is superb at ISO 100, who cares?
What about waiting for a 5Diii to get a better focus system on the 5D and some of the other 7D feature set. The rumors are that the 5Diii will be a 28 mpx camera and that we would not see it till very late 2011 at the earlies. Given that the 7D has no on-camera flash, it is not likely to inherit the nice 7D feature of being able to remote command off camera speedlights. If all this is true, it is sad that the pixel count wars continue. The 7D with 18 megapixels is close to the resolution performance of all but the best lenses. The 5Dii with 21 megapixels is more than I need, what is the point of 28 megapixels (other than to keep up with Nikon announcements).
In searching on the web, I found an extremely instructive pair of images shot by a fellow by the name of Ben Jacobsen and posted on the Canon Digital Photography Forums in August of 2010. One photo was shot using the 5Dii and the 24-70L lens. The other was shot using the 7D and the 10-22 lens. Both were shot off a tripod with long exposures. Given that I own the 10-22 lens (and like it a lot), this comparison is exceedingly relevant to my thinking.
Here are the images, first the 5D (on the left), then the 7D.
Certainly it is impossible to tell any difference on the images rescaled for the web. To really see a difference it is necessary to download the full sized images and look very closely at detail in the rocks and the far shoreline. My impression is that you would be hard pressed to care one way or the other, even with a fairly large print.
A person pondering the 5D versus 7D decision, might well ask themselves what other piece
of equipment they could buy with the $1000 they would save by purchasing the 7D.
You could buy:
Tom's Digital Photography Info / tom@mmto.org