So, I'm leering on the edge of a 70-200mm and I wanted to know what people thought. I'm by no means a pro but am definitely critical of IQ. So has anyone done a shootout of the Sigma vs. the Canons (MKI or especially MKII). Sigmas have never treated me wrong, but everyone raves about the MKI/MKII. I can't rent a Sigma around here, so do any of you have real world experience between the two? Cost of the Sigma is obviously a factor since it's half the MKII.