I should know as I own two 5DMarkII's and a 7D. I bought the 7D thinking it would be close to the 5D in image quality, or at least in the same ballpark. I was excited by its features and thought I could live with a small drop in IQ for the sake of all those other goodies. Unfortunately it's not even close.
I get so beyond tired of this nonsense. I have made 24" prints from the two cameras and posted 100% crops online, and in both cases asked viewers to tell me which came from which camera. Note that I did not show them labeled views, but unlabeled ones. Not once has any viewer successfully done this.
Over a dozen print views and well past a hundred online views and not one person could even guess correctly.
At high ISO, there's a difference. At low to mid ISO they're not only close, they're so close that it's impossible to tell them apart in a real test. I regularly make large prints and I consider paper choice to have a greater impact on final IQ than the choice between these two sensors.
If you show a human being two identical prints...same exact image...and tell them one came from a model A camera but one came from a much better model B camera, the average viewer will tell you they see the difference and the B print is much better even though the prints are identical.
Some will even go into great detail as to why B is better! This is human nature. It's the inescapable way our brains are wired. The true test is to show unlabeled images and ask which, if any, is better, or if they are the same. And guess what happens in those tests...
1. You have a fantastically featured awesome little camera with great autofocus, button layout, flash trigger capabilities, weather sealing, responsiveness...yadda yadda yadda.... but with a horrible sensor
The sensor is fantastic and is better for those in focal length limited situations.
That lineup is just whack and they should really do a sensor swap. Canon will address this by putting crop sensors solely back into the XXD line with the 70D, and boosting the features of the 5DmarkIII so they meet or surpass the 7D - guaranteed. Well shots per second *might* be down a bit, and there will be no onboard flash but these are the only exceptions I can think of).
That and pixels on target. I use a 7D heavily for sports photography, and I do not want to sacrifice the crop factor or a single frame per second. I have numerous 24" prints that involved more cropping with a final image size of 9-10 MP. On a 5D2 or a theoretical 5D3 at 22 MP I would have ended up with 3-4 MP, not enough for the print.
Canon needs a pro crop body geared for sports. The 7D basically filled that role. Perhaps a 70D would as well, but my fear is that they would cripple it somehow to make it clearly lower in the lineup than the 5D3. Which would put us back to the days when Nikon had the only pro crop body.
A 61pt AF 22 MP 5D3 will compete against the 1Dx, but not the D800. I can tell you right now if Canon doesn't have an answer for the D800 this year at a comparable price point, I will have a D800. The 5D2 was successful because it was squarely targeted at wedding/portrait/landscape/studio work with 21 MP and high quality video at a much lower cost than Nikon's 25 MP body. Now Nikon is offering those users 36 MP at a comparable price point, and I know one pro wedding photographer who has already ordered one even though most of his current equipment is Canon.
Put simply, for sports work I want fast frame rates and crop because I'm typically FL limited. For landscapes I want MP, MP, MP. Canon could possibly fill both roles with a FF body that offered 8 fps and 35 MP or more (more would be better). But if they do that at the D800 price point, then what's the point of the 1Dx?
If your 7D ego has just been bruised then I suggest you get yourself a full frame camera and start pixel peeping and see the "full frame light" my friend
Been there, done that, didn't see the light, and neither can anyone else when put to the test. I hate pixel peepers any way. I make prints. I don't sit in a dark room all night studying images for flaws at 400%.