My colleague at work has one and takes some great nature/wildlife shots with it.
Ive only really heard good things about it. However, I think these comments tend to take into account the relatively cheap price of the lens. Its no pro lens, and is rather slow.
And to people who say it is heavy, i say pah!

My sigma 120-300 2.8 weights 2.6kg compared to the 1.8kg of the Bigma. My Sigma is way to heavy tho!
I'd say for the price its a good buy. Just be aware of the speed.
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My Photography D70, 18-70, 50 f1.8, Sigma 105, Sigma 120-300 f2.8, 1.4 TC