Summary: I bought this lens in Aug 2006, I was looking for a zoom to shoot youth baseball , soccer and to sub as a wildlife lens also.I tried the 70-200mm's, both Canon and Sigma , but they where to short for my needs. I then ran across the Sigma 100-300mm f/4 EX lens. It was equal to Canons 70-200mm's in Image Quality and AF speed. When a tc was added to the 70-200's, the 100-300mm out preformed them. The AF on this lens is very fast and accurate. Image Quality is excellent.
Pros: Durable, Quick Focus, Rugged, Sharp Focus, Strong Construction
Summary: This is an update from my previous posting... OK, I must now elevate this lens to the status as the worst lens I have ever owned. I got my first camera and lens in the 1960's so that says something. This lens has been back to Sigma twice, both times for focusing issues, and it continues to hunt, even when I am not in AI Servo mode. The number of out of focus images is at least 25%.
Summary: I'm using this lens for a year now on my 30d and it gave me many many good shots.Optical performance is excellent right from f4 and I found that f5 is the best (fast enough, and top optical quality through the focal range). I noticed a little CA at the long end at all apertures, but it is easily removed with Photoshop or other post processing software.Construction is top quality, autofocus lighting fast and accurate, manual focus is a joy (so smooth) and it seems that...
Summary: I have had this lens for just a little more then 2 years and have had it out of my camera bag 5 or 6 times in the last year. Sigma to repair focusing issues twice in the first year. The first repair because it failed to provide proper focus about 75-80% of the time and the 2nd time due to excessive hunting. Even after the 2nd repair, the lens will still hunt, even tripod mounted and focused on a stationary object.
Summary: This lens gives you f/4 300mm with the flexibility of a zoom. The HSM is blazing fast and the IQ is on par with if not better than the 70-200L. I use the Sigma 1.4xTC with this lens all the time and it is a great combo. If you're shooting daytime sports and can't afford a 2nd body and f/2.8 glass, this is the lens for you. Drawbacks are that it is big and heavy (but lighter than the 70-200L IS)and doesn't have IS.