UPDATE: 10/10/09
I know there is a lot of interest in this lens from people out there wondering if they should splurge for the Canon or get the Sigma. The Canon IS version is obviously the best choice because it has both the f/2.8 aperture and IS, but the reality is that not everyone can afford one right away. For those looking for an f/2.8 70-200 zoom, I think that the Sigma is a great option. Here are my initial thoughts after using it for a few days. I will continue to update as I use the lens more.
Build
I really like the build quality of this lens. It feels very solid in your hands and the ergonomics are really good overall (in my opinion). Both the manual focus and zoom adjustments are dampened very well. One of my favorite things to do with the lens is leave the tripod collar on when shooting hand held. If I palm the tripod collar, my ring finger falls directly on the zoom and I am able to zoom using one finger. Very smooth, easy and convenient. I have used the Canon 70-200mm f/2.8 IS and have found the Sigma to be very similar in this regard. I prefer the finish of the Canon's to Sigma's, but it's a minor nitpick. The Sigma finish can get dirty and scratched up easier. The lens hood can also be a bit of a hassle to get on, but it's not really a big deal. I will also say that Sigma has the best tripod ring that I have tried. I love how it has marks for both portrait and landscape for proper alignment as well as the hinge which allows you to remove the tripod ring with the camera still mounted to the lens. With some practice, I could probably take the lens off of a tripod and pan for a a bird in flight in a matter of seconds. Very cool I think!
Focus
I love the autofocus on this lens! Coming from lenses like the Canon 50mm f/1.8 and Tamron 17-50, having HSM focusing is a handy change. I can do without it on shorter lenses, but feel that it adds a lot more versatility to a telephoto, allowing me to capture action shots of wildlife as well as take sports shots.
Focus acquisition is nearly instantaneous in all lighting that I have used it in. (I have shot some early morning shots, but nothing too extensive to test this out). It does struggle a bit focusing on things in difficult situations like branches, through weeds etc., but this has happened with every lens. After a few tries, I can usually lock on. It might be the best lens that I have used in these situations. (I haven't tried the Canon in this situation).
Focus tracking is also very smooth. It maintains focus without any of the jittery nonsense that comes from non USM/HSM lenses. It tracks moving objects well and I haven't had any problems with going in and out of focus. I think that it is VERY close to the Canon in focus tracking, if not just as good. I can't really tell the difference, but I have limited use with the Canon so it could be slightly better. The Sigma does a great job regardless.
Here are some quick examples I took the other day:
 | HTTP response: 404 | MIME changed to 'image/gif' | Byte size: ZERO | PHOTOBUCKET ERROR IMAGE |
Sharpness at f/2.8 and 200mm.
This is probably the most thoroughly discussed issue with this lens versus the Canon. Frequent problems with focus calibration have not helped out with this issue at all. I think that a lot of the sharpness issues that people have relate to front or back focusing. Having said that, there is softness and a lack of contrast wide open and at 200mm. This is a slight difference between other focal lengths and is nothing that can't be fixed in post processing though in my opinion. Comparing to my Tamron 17-50, the Tamron is noticeably sharpen wide open at all focal lengths.(Please note that the Tamron is a very sharp lens for a zoom). Everything sharpens up nicely by f/4, where sharpness isn't even an issue. I have found the results at f/2.8 and 200mm to be easily usable, which is good enough for me. Here are a few examples, both at f/2.8 and at 200mm. (these also are from photobucket, so will most likely suffer from softness due to poor compression.)
About a 50% crop
| HTTP response: 404 | MIME changed to 'image/gif' | Byte size: ZERO | PHOTOBUCKET ERROR IMAGE |
about a 25% crop
Macro
The Macro for this lens is decent for a lens of this range. I found it acceptable, but would want to use extension tubes in order to get closer to actual macro range. Here's an example of what you can expect from Macro. This was taken near minimum focus distance at 200mm f/2.8.
| HTTP response: 404 | MIME changed to 'image/gif' | Byte size: ZERO | PHOTOBUCKET ERROR IMAGE |
Ratings: Here is how I would rate the lens versus the Canon (in my experience) on a scale of 1-10.
Build
canon Sigma
10 9
Focus
10 10
Sharpness
8 7
