The old version of this lens is my workhorse. 80% of my work is shot with the version I. After 7 years and 500,000+ images taken with it I thought it'd be a good idea to update. I recently updated my 70-200mm to the version II and was blown away by how much sharper that lens was to my IS version I. With the 24-70mm update I was hopeful for a similar giant leap in sharpness. I was disappointed. I did a tests through the max, middle, and minimum apertures and zoom. I did a side by side comparison with identical images from the vI and vII and asked people to pick the sharper image. It was inconclusive. After spending $2400 I was hoping for better. The CA looked slightly improved. (slightly!)
As a professional with assistants I could care less about the weight of a lens. I actually prefer heavy (steadies the caffeine shakes:) But, if you are a PJ or a hiker on the go it's light weight and shorter length is a plus.
Build quality is good, but has more plastic than the old one. I'm very grumpy about the move to the 82mm front end.
I realize this is pointless to say, but the new lens hood looks wimpy and dumb. People were always impressed with the looks of the old one:)
If your a budget minded hobbyist I'd skip this lens and try to find a low mileage vI. For rich guys and pros buy it, but don't expect to be blown away. I'm only keeping mine cause my vI 's barrel is wearing out.