Sigma 50-500mm f/4-6.3 EX APO DG HSM for Canon AF

This is a very nice and convenient lens, combining the wide enough 50mm with the 500mm telephoto. Images are quite sharp, especially under the 200mm, after which sharpness is less. Autofocus is above the average, fast enough for some action pictures. Another benefit is the Hypersonic Motor (HSM) autofocus system (an equivalent of the Canon USM), which provides rear focusing, without lens extension and external movement. Now the bad news: It is heavy! After a day of use, you will definitely remember that you were using it! Use of a monopod will help you a lot.

Conclusion: It is a very honest selection, cheaper from the Canon EF 100-400 f/4.5-5.6L IS USM and with wider focal length range, but with lower image quality, especially in high focusing distances. If you are a serious amateur who likes action photography and you don't want to spend more, this is the right choice.