I have read some reviews that are all over the board on this lens, but my experience should help sway you if you are on the fence about buying one.
My copy is sharper than THREE different copies of the much ballyhooed 70-200L f4 that many consider to be one of Canon's sharpest pieces of glass. It is also sharper than TWO different copies of the 70-200L 2.8--one of which I currently own. I wouldn't say it is sharpER, but at least AS SHARP as my 24L 1.4.
Did I mention this lens is sharp?
Color rendition is superb resulting in very vivid scenes. Distortion is minimal at 10mm and gone by about 14mm. I haven't noticed any flair (but I do have a hood) and my copy has no purple fringing to speak of.
My only beef is that it isn't a constant 2.8. If it were, I would have gladly paid twice what I paid.
I recently took a vacation in which I had my usual 30lbs of gear in tow. After a week and close to 500 shots, it dawned on me that every single picture I took was taken with this lens.
Bottom line: if you have a crop bodied camera, you will most likely want this lens. I know not everyone likes to shoot wide--I didn't used to either--but this lens opens a plethora of creative possibilities that I didn't even know existed before.
Get one, you can thank me later.