Bought this lens today, I wanted something lightweight and good in situations where I need to handheld in low light.
Tested on a 5D MKIII, the first copy was perfectly sharp in the bottom corners, but the top corners were 'merely' good (very slight fuzz). The second copy I test was simply perfect, very very sharp all over. The serial number is 90100000001, so maybe this lens got some extra love because it was the first in a production run.
The image is as sharp as a 50mm f1.4. I have some tiny issues correcting Chromatic abberation, but this is because I do not have CS6, so I am missing the correction profiles. I expect CA to be completely removed with the right correction profiles.
Image stabilization works very well on this lens, I managed to get consistent sharp shots with shutter speeds of 1/10 sec (it is a wide lens of course).
I agree that this lens is (severely) overpriced, but for me it is worth it as it offers me very high quality hand held shots in low light.