It's still way to early for me to tell. I've only shot Macro with it so far and the differences with it and my t2i in overall IQ is subtle just after 3 0 minutes of shooting. I've lost reach, the DOF is much narrower which is good and bad for my macro shots. I shot some iso 3200 shots and they looked amazing in the viewfinder but ended up looking really flat in DPP and LR. I have everything turned off except for Chromatic aberration and illumination and currently shooting in Standard mode and have strictly shot center point AF. The below image was shot with the 100mm 2.8L Macro: Wide open at 2.8, 1/200, and iso100--sharpening and NO-NR
I am looking forward this weekend test driving with the 50mm. You can see one of my floral keepers here:
I also have T2i and 5DIII, and another way to look at it is T2i is a bloody fantastic camera! After all, 7D, 60D share the same sensor as T2i.
My biggest gripe with 5DIII so far is how the magnification button was moved from right upper to left side. This makes it much more difficult and fussier to manually focus with left hand with manual focus lens (such as TS-E or MP-E) while hitting the magnification button with right finger (no longer able.) I have to either take my left focusing hand off the lens focus ring and push the mag button or use my right grip hand to hit the button. This was a big backward step IMO.
*EDIT* Figured out how to customize the "Set" button to function as Magnification button. While not ideal, it'll do for now and much better than left-sided button.
Another oversight was how one is no longer able to assign flash exposure compensation to a assignable button. Even T2i could do this.
At any rate, I like the flower, but were you trying to shoot wide open instead of traditional macro f/stops for some reason? Below is my 5DIII, 100L shot at f/8.
DZ3C0245 by drjlo1, on Flickr