DIYPhotography has completed an early review of the brand new Sigma 100-400mm f/5-6.3 OS Contemporary lens. At $799USD, this lens looks to be an absolute steal. If you're not all that interested in buying the $1999 Canon EF 100-400mm f/4.5-5.6L IS, this new Sigma may be what you're looking for.
From DIY Photography:
Bottom line, I like this lens, it has plenty to offer and for it’s intended price point, I think that offering is a good deal. All the operational aspects of the lens met or exceeded my expectations and did so at a weight and size that are surprisingly less than I would have expected. Image quality is solid, with a minimal number of optical issues (quite frankly less than I expected for the range of focal lengths available) are minimal. I do wish it had a tripod mount, as I could see using this on a monopod for sports, when paired with the right body, but that’s not a huge knock, because the weight, size and Optical Stabilization do seem to offset that need some. Read the full review