You need this for near action on C sensor digitals

I started looking for a lens with more low end awhile ago. My primary lens that I use most of the time is a Canon 17-85 IS, that is the perfect lens for what I use it for 90% of the time but sometimes, if the action is really close even 17mm with the 1.6 magnification factor for the C size sensor is too much on the low end (works out to about 27mm).

I am always hesitant about buying non-canon glass because they change their lens protocals sometimes in an effort to frustrate the use of third party products with their cameras (if you ask me they should be chased down and hung for that practice but that's just my opinion). I have owned sigma glass in the past with my film cameras and while I don't think they've ever been considered particularly top end lenses their products are not bad and the price is right. Sigma has always included lens hoods and cases with their glass...this is another selling point because Canon doesn't and some of their hoods cost over thirty dollars even on ebay (if they can even be found).

The canon consumer glass with nearly the same zoom range is over two hundred dollars more than this lens and doesn't get very good reviews on the camera sites or in the forums. The Sigma EX line has the reputation of focusing as fast as canon consumer lenses and this particular lens has a pretty good reputation for sharpness etc. on the forums and review sites. Sigma makes at least one other flavor of this lens with nearly the same zoom range, the 12-24mm, for whatever reason that lens is more than this one and doesn't seem to get that much better reviews.

What's in the box:
lens
case
hood
lens cap
tail/rear cap
warranty materials

What else do you need:
Any photographer will tell you the first thing you do when you buy a new lens is get a UV filter for the front of it. This filter will protect your front lens element from accidental bumps, stupid child fingers and other things (sand storms, liquid splashes, etc.), that can destroy your lens. At 77mm these are very expensive filters, you can do better on ebay than in a camera store for this bugger, there are some really cheap filter products on ebay that will affect the quality of your shots negatively; stick with a brand name such as HOYA, Vivitar, etc. If you use any special white balancing filters, like I do, invest in a step up/step down ring so you can use it on this lens.

Operation:
Like the canon lenses there is a Manual Focus/Auto Focus switch on the barrel of the lens.
Like the canon lens there is one ring for focusing and one ring for zoom.
This lens seats on the camera like any other

Note: Sigma reverses the Focus and Zoom ring on this lens. It takes a bit of getting used to because the Zoom ring is in the back on canon lenses while the focus ring is in the rear on this lens.

I have not read anywhere that this lens offers full time manual focusing like the Canon lenses with the USM motors offer. I rarely use manual focus so I'm not really that concerned about it.

What I think of it:
I have shot with this lens once, I was shooting Karate in a gymnasium(took about 100 pictures with this lens). This kind of shooting needs fast accurate focusing, so far I have no complaints that the lens could not keep up with the camera (even at the 5fps the 20D is capable of shooting). I tend to agree with most reviews that the lens is a little soft at max aperatures, so the cure for that is to use a smaller aperature if you can manage that. When my school has belt tests they tend to set the mats up ten feet in front of the bleachers so with this lens I can sit on the floor and easily catch all the action going on right in front of me, with my other lens sometimes if somebody tossed someone over themselves it was too close to the top of the frame and something got cut off. I think this lens is fairly bright I'm happy with the colors I'm seeing and at least so far I'm not noticing any problems with softness near the edges or anything like that. I believe that the way the rear of this lens is designed prevents it from being used on a film camera, it's for digitals only and I'm also not sure if you can use this on anything but a C sensor camera, i.e. I don't think you can use this on a 5D or any of canon's other higher end bodies (i.e. you're limited to the Rebel, the Rebel Xti or the 20D, 30D...I don't know about the upcoming 40D if it's also a C sized sensor it should work on that body but I've heard rumors it may not be).

For what this lens costs versus the quality, you are getting your money's worth. If you shoot weddings you will want a wide angle lens such as this one, it's great for shooting the whole wedding party or those priceless shots of the bride with her humongous train (of her dress) spread out behind her. This should also be a great lens for landscapes and the ocassional artistic building shot.

I'm not sorry I bought this lens. If canon should hose me by changing camera focusing protocals on the body, most lenses can "rechipped" a certain number of times. I have no idea what might be charged for that service but it's undoubtedly cheaper than a whole new lens. My only complaints about the lens are that it's a bit stiff when changing focal length (mostly because it's new I'm sure) and that they put the zoom ring on the front of the lens so it's backwards from canon's products (annoying but you get used to it). I love it, not going to return it. Now I just need a second body so I don't have to go nuts re-shooting the white balance when I change lenses and I can have both lenses available as needed.