In most of the reviews I've read, everyone has been suggesting to save up for the Canon version. The problem is that Canon has multiple versions of the 70-200mm f/2.8, the non IS, IS, and IS II models.
I'd be buying them used, and the non-IS model used is the same price as this sigma. All the comparisons I've seen have been between this Sigma version and the Canon IS II model, and they review it to be pretty close.
At this point I'm deciding between the Canon 70-200mm f/2.8 IS used (~$1300) versus this Sigma version ($1000). The Canon 70-200mm f/4 IS is still available locally and looks pretty tempting at $800 almost new. I shoot mostly portraits at the moment, BUT have booked some upcoming weddings in the year. Will I really miss out on the 1 stop difference?
I'm no professional by any means, just another amateur looking to get the best bang for his buck.
Thats just how 3rd party/generic lenses make money. Many are very favorably comparable in quality to name brand lenses, with enough research you can find reviews that back up those claims, and there are a few 3rd party fanboys that lurk forums that will tout their superiority till the end of time. They are also priced to compete with name brand used and refurbished lenses, and always cheaper than the new name brand lenses. When a potential buyer like you is looking for a specific lens, they are always tempted with the dilemma of "hey I can get this off brand lens for just a little more than a used Canon version, and some websites rave over how good this lens is in comparison with the Canon too, so it makes sense to buy the new off brand one right?" The problem comes with holding their value. A decent Canon or Nikon lens always holds its value very well. 3rd party lenses never do, there is no demand for them compared with name brand. None of these lenses are inexpensive, it comes down to wise investment of your money. Buy 3rd party lenses used never new.