At the time I couldnt afford the lens I really wanted which was the Canon 100-400mm IS L, so I opted for the Sigma 120-400mm at less than half the price of the Canon but with excellent reviews. I was VERY happy with it, it performed really well, but did require good light to be at its best. I at last could afford the Canon I wanted, so I sold the Sigma, the difference is amazing. The canon knocks it into a cocked hat! Same aperture etc.. but so fast, quiet and lovely!!
My advice, get the best you can afford.