While this lens will allow you to shoot in very low light, the resulting images will be very soft.  It sharpens up nicely by f/4, but there are several zoom lenses (e.g. 24-105mm f/4L) that can match it at that aperture.  While quick, the auto focus is not very accurate.  I spent a lot of time dialing in the AF micro-adjustment on my camera (7D), and while I was able to improve the average case, the variance was just too large to yield an acceptable "keeper" rate. 

Question: How would you rate the sharpness of the image with the lens?
Answer: the resulting images will be very soft .   It sharpens up nicely by f/4