I bought the MKII from Kerso on here and I have to say, the lens is simply amazing. I can't compare it to the MKI but the speed of the AF and the crispness and clarity of the final result is fantastic. I came into a little bit of money recently and was dithering about which lens to buy when I saw the reviews of the MKII lens. It is expensive but even with my very limited skills and technical knowhow this lens has been a revelation to me. This is my first L series lens and as far as I am concerned, worth every penny!