I've never shot with the Sigma, so I really can't comment on the optics. In general user reports seem to be pretty favorable. I did test the
Canon 100-400/4.5-5.6L IS a while agao and I was impressed with its optical performance. However at around $1600 it is quite a bit more expensive than the Sigma zoom.
One thought though. If you're shooting birds you may need all the focal length you can get. If that's the case perhpas the
Sigma 150-500/5-6.3 OS might be worth a look. It's only about $100 more than the
Sigma 120-400/4.5-5.6 OS and it's $600 cheaper then the Canon 100-400. Again user comments seem generally favorable. It won't be as sharp as a 500mm prime from Canon (or Sigma), but the Canon prime is over $6000 and even the Sigma is $5000.
The 150-500 is only 1/3 stop slower then the 120-400 and having 500mm will be an advantage, especially for small birds. You could add a 1.4x to the 120-400, but you'll lose AF and likely the image quality at 500mm will be lower than with the 150-500 (this is a guess, I haven't tested either one).
I do like the
Canon 70-300/4-5.6IS but it is a bit short for most bird photography. It's sharp, small, light and relatively inexpensive, but 300mm may just be a little too short for you.
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well I got myself the Sigma lens for Christmas. Actually my wife surprised me and she bought it. So far i am very happy with it. It will take some getting used to the focusing and image stabilization. But check out my images at my Flickr site (3kids2dogs). 