I own the 77, 21 and 50mm primes but frankly the 12-24 is a more consistent lens than all of them. It's sharp at f/4, sharp in the corners a stop down with outstanding colour and contrast. I have found CA's to be minimal, my only real complaint is that if the hood is not screwed on fully it intrudes on the image at 12mm. The hood is large and this makes the lens hard to carry (it's not a big lens sans hood) but with this FOV you have to use the hood.

Flare is very well controlled and build quality is first rate with snappy AF and quickshift. This is what all lenses should be like.