I really wanted to like this lens. I bought it to compliment my superb 70-200 IS f/4 L lens, and thought the two would make an excellent combination for travel and low-light shooting. Reviews here, at SLR Gear, and PhotoZone were most promising. I thought the price was almost reasonable if the lens would deliver quality equivalent to my 70-200 IS L.
The build quality was fine - not quite "L" quality, but better than my 10-22 zoom, and similar to my Canon 100mm USM macro. I didn't have the lens long enough to comment on dust problems.
Over about 150 shots, I found the AF to be inaccurate on my Rebel XT. (I have had some issues with the EF-S 10-22, as well, but with no other lenses, including the Tamron 28-75 f/2.8 and Canon 100 f/2. Maybe my Rebels don't like EF-S lenses?) I thought perhaps this f/2.8 lens would benefit from the "precision" focusing system on the XTi, supposedly the same as on the 30D, but results were no better. At wider settings, AF performed worst, being accurate about 1/3 of the time. At 17mm, the lens consistently showed 3m for large objects at infinity, and EXIF data showed 3.27m. These distance data were for larger subjects, with good contrast, in good lighting (although IS was always used). At the longer settings, AF was better, but still not quite good enough to be successful 2/3 of the time.
I quickly lost confidence in this lens on my camera bodies. I then read a review at kenrockwell.com, and he reported AF problems with his copy, too. Because of that, I decided to return the lens for a refund rather than exchange. No matter how sharp a lens may be, it is of little value if it cannot be focused accurately. This is a premium EF-S lens designed for APS-C bodies, so I expect accurate focus at least 90% of the time, and expect AF to be better than my ability to manually focus in most normal situations.
I would have been very disappointed if this had been a $400 lens, but I think thousand dollar lenses should not be shipped unless they are in perfect calibration. I sent it back to B&H Photo, who were very friendly and helpful with the return.
I am resisting giving the lens a very low rating, because so many others are happy with their copies and I don't want to overly skew the average rating. However, I can't give it more than a "6" or "7" due to the erratic AF problems and poor quality control. For $1000, I expected much better.
Like I said, I really wanted to like this lens. It's the first one I have had to return in many years, and the first one I ever returned that cost more than $500.