has anyone used this lens? I can get a reasonable deal on a canon 100-400 IS but was wondering if the extra reach of the Sigma 50- 500 might be better for wildlife. I have looked at the Canon in a few stores and all the info I have read about them seems good and the Canon is a f4.5-5.6 compared to the Sigmas f4.5-6.3, would this make a great deal of difference?