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This is my all around most useful lens due to the focal range, image stabilization and light weight. It is NOT the best lens I own, but when I wish to travel light and the primary purpose of the trip is not photography, I can pack my 30D, this lens and my EF 50mm f/1.4 for low light, all in a small camera bag that fits inside my roll-on, and be well prepared for almost any photographic opportunity.

The OS function works extremely well, but I think it is generally slower to engage and obtain full stabilization than the Canon IS function. I do not think this substantially affects the usefulness of the lens in most situations, but you must take this into account to get good results with the lens. Also, once it is engaged, the Sigma OS function may be slightly better than the Canon IS function. The Sigma OS appears to enable you to shoot 3-4 f-stops slower versus 2-3 f-stops for most Canon IS lenses.

My copy is not the HSM version. The AF speed is not as fast as USM/HSM lenses, but it is generally more than adequate.

My results with this lens are sometimes inconsistent. And I haven’t really figured out why, even though I have been using this lens for almost a year. I think I sometimes forget to give the OS function time to fully engage and this results in image softness, but not outright image blur. The bokeh for this lens is not really very good, so images that make strong use of bokeh can be challenging or the results disappointing. Also, the image sharpness, especially in the corners, drops off pretty dramatically at the lens' largest apertures, and especially at the widest focal lengths.

Overall I am satisfied with this lens. I have recommended it to several friends and at least two of them have purchased the lens and are happy with it. Its usefulness is mostly that it performs adequately in all of the ways it is intended to. This makes it a good choice for those times when you can’t or do not want to carry a bagful of lenses, or for when you need to frequently and quickly switch from wide angle to telephoto and back.