These are not test shots, archiecture, or otherwise shots where I was trying for maximum sharpness but I shot with this lens on my 5D almost exclusively when I went to the US Cyclocross National Championships in Rhode Island in December. (Type of bicycle racing.)
I found it great for this. Honestly for more artsy stuff and non-sports I probably wouldn't even think about using the zoom over my primes, but for this I found it awesome for a sport like this where you can get extremely close to the athletes.
These are all funky shots, lots of motion blur, etc.. but lots of them are wide open or close to wide open and they have not been edited much, the levels of light fall off that people complaining about are clearly not evident in mine if these are any indication. (I mean people were complaining about 2-3 stops of light fall off in the corners, visible even down to f/8, totally over the top claims for such an expensive lens)Cyclocross pictures - mostly 17-40 f/4L
Also this was completely stopped down to f/22 to get the blur so it is no indication of corner issues wide open but this is probably one of my favorite shots I've taken since I got my 5D, and I shot it with the 17-40 @ 40mm. I've printed this at 12x18 and had it framed as a gift, and I've got another 13x19 print for myself, and there are absolutely no sharpness issues in the corners on these prints. (And surprisingly it doesn't seem like diffraction was really a problem)
The corners were burned in photoshop so don't assume the dark corners have anything to do with the lens.