I own a similar one. The Sigma 70-300mm APO DG Macro which you can find a review for here: Sigma 70-300mm F/4-5.6 APO DG Macro Interchangeable Lens Review
For $300 I found it to be an excellent lens. Mind you, I am comparing it to my kit lens. I still love this lens though. For my budget it gave me a wonderful zoom capability on my XSi and a pretty good quality. All of the shots in my Polar Bear Dip set were shot with this lens from 65 meters away from the action. Here's a link to the set so you can see the results: Herring Cove Polar Bear Dip - a set on Flickr
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