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There is a review of the older version of this lens from Gordon here
. He also compares it to the Nikon and Tamron variant.
Sigma's latest incarnation of this lens, the Sigma AF 18-200mm 3.5-6.3 DC OS HSM II has optical stabilization (very helpfull at longer focal lengths) and HSM, so will focus on your Nikon 5100 body.
Unfortunately I can't find a decent review as this model was just announced. But its immediate predecessor AF 18-200mm 3.5-6.3 DC OS HSM has the same features plsu you can find a review there
Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews
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