A friend of mine has the Sigma, and is quite happy with it...

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Canon 40D w/BG-E2N
Canon 20D w/BG-E2
Canon 70-200mm f/2.8L
Canon 24-70mm f/2.8L
Canon 85mm f/1.8
Sigma 17-70mm f/2.8-4.5
Sigma 50-500mm f/4-6.3

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