Oh man, I feel really silly (my apologies) regarding my post above. I got this thread mixed up with the thread discussing Sigma's 200-500 f/2.8
Bradley clearly did the same thing
So, yes, this is discussing the much smaller, cheaper, Sigma 50-500 f/4-6.3 lens, NOT the new, huge, Sigma 200-500 f/2.8.
Husain, I have not heard great things about the 50-500, and, to be honest, unless you want that zoom range, you'd be better off with two or three more dedicated lenses.
Canon's 28-300 is much much better optically (from what I've seen, sorry, no proof to post) and even then, you're in a bit of a compromise. And the Sigma has no Image Stabilisation, and a slow 500mm lens with no IS is very difficult to use in all but the best of light. Reviews such as
this one say that the 500mm is rather poor beyond 300mm - so if that's where you're going to shoot, much rather get a Canon 100-400mm, or a 400mm prime (even the 'el cheapo' 400mm f/5.6L - much, much better, smaller and lighter than the Sigma). Or, staying with Sigma, their 80-400mm is also somewhat better optically, and it does have image stabilisation.