I decided on the f/4. I was against the f/2.8 because it's twice the price, twice the weight and a fatter lens. (Then again, you need more glass for f/2.8...)I've been very impressed with the image quality. Every image I've taken has been razor sharp, with no noticeable CA.
Fast focusing; sharp; very lightweight; cheap alternative to the f/2.8 version if you can live with f/4
NONE