I picked up this lens from local craigslist today. First impresson is very positive so far. It actually feels smaller than its dimension suggest. It is as long as 24-105L retracked, but most of the body is much narrower, only the front thread are suddenly increase in diameter. The build quality is up there with other L series, but I think it is one step better than 24-105L due to its partial metal and one-piece construction. It also feel much lighter and easier to handle than 24-105L. IQ wise, I am happy with the few test shots so far. Out of camera color seems very good, especially red saturaton. Contrast? I am not so sure yet, the sky is over cast, so everything looks dull. Sharpness seems to be good at center. I cannot comment on edge and corner yet. One negative aspect I notices is vignetting at 16mm, even without any filter, it is very noticeiable even on the lousy 5D LCD. Maybe that is the way ultra wide zoom is. I cannot wait until weekend for some more photo shoot with this lens
