Highly prone to flare with CA, very soft feel. Focusing is slow at best.
I have this lens to accompany my 10D cameras and 1Dm2. Its a different lens as compared to the 20mm Canon that I also own. Its softer with a very nice background blur. It performs best under even light as it is very suceptible to flare. Just forget pointing it at a lightsource and holding edge sharpness.
Don't count on focus tracking either. The motor is a bit slow compared again to the USM in Canon lenses. But, the macro capability combined with the nice smooth 'bokeh' areas make this a pleasant specialty lens. I really like 'look' of it when appropriate however. I use it in portraits and detail shots in available light. When used close to the subject the short depth of field becomes more dramatically apparent, and the wide angle feel is somewhat unexpected.