I originally bought this lens a month or so after it came out. I spent a month lusting over it and Canon Price Watch had a great deal on it from one of their street price deals. This review will compare it to my 35 Art, because I had that first, and was hoping to get something akin to it.
I have a bit of a love/hate relationship with it. I've thought about selling it a few times, and each time I want to sell it, I'll go use it and remember why I keep it. It really excels as a museum lens. 24mm on my 6D is the perfect focal length for going to museums for me. I also use it a lot for landscapes. There's no vignetting with my LEE 100mm filters.
This lens is very sharp wide open, and gets noticeably sharper as I stop it down. I do minimal sharpening and am always happy with results. It does taper off in the corners, even stopped down. It is not as sharp as my 35A, but I am happy with the sharpness.
The autofocus on this lens is is pretty solid, but a step below my 35A. It is a little slower to focus than the 35A and I cannot get focus in as dark of conditions.
Chromatic Aberration / Coma / Distortion / Vignetting
I don't notice much chromatic aberration with this lens, and anything that is there gets cleaned up nicely by the defaults in Lightroom. Coma is pretty bad in the corners, almost to the point where it is unusable for astrophotography. I go back and forth with this lens and my 35 Art for astrophotography. This lens has much worse corner performance than the 35A, but it's much wider, so I go back and forth. I don't notice much distortion with this lens that isn't easily corrected with the Lightroom defaults. Under f/2.8, vignetting is pretty severe, at around 2+ stops, which is enough that it's hard to correct in dark images (like astrophotography).