I have the Sigma 17-50 and Tamron 70-300 and I am very happy with both. I cannot compare them to the comparable Canon lenses but both of these lenses are quite happy on my 7D. From the Sigma; constant F2.8, fast and accurate hypersonic focusing, Optical Stabilization, lens hood and a lens case for under $700. From the Tamron; fast and accurate Ultrasonic focusing, very effective Vibration Control, lens hood for a final, after rebate cost of $350. Both of these lenses are in my bag for less that the Canon 17-55. At this time (a single income family), these were the best options for me and I am very satisfied with my lenses. So much so with the Sigma, that their 70-200 f2.8 may be next, unless Tamron updates theirs, and then I'll have a tough debate.
These Third Party lenses are great options and have made it possible for me to get some high quality glass without totally breaking the bank. I would probably not have a constant f2.8 if not for the Sigma--a great option.
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