The latest issue of the British Journal of Photography has just reviewed this lens. Have a read of it at http://www.bjp-online.com/public/showPa ... age=866117
The Sigma is no more expensive than the Tamron despite the HSM focusing versus a plain micromotor. Build quality feels heavier, an extra 200g in a 10mm shorter design. Coming with a rugged tripod mount (1/2 inch thread only), which is removable and doubles as a small handgrip, the Sigma also has a good deep lens hood and a padded case with strap.
It would be hard to say it's much inferior to the branded equivalents. Optically, it doesn't have the same wide open central resolution as a Nikon or Canon at long distance and the long end of the zoom, but it can be better at close focus distances.
The HSM focusing seems designed to leave Canon owners - those most used to this tech-nology - happy with an equally fast, silent equivalent.
Weak points are that it loses resolution and illumination towards the edges of full frame at 120-200mm when focused beyond portrait distances, and f/8 is needed for critical landscape work with detail corner to corner. Within the 70-120mm range, it performs well at f/4.
Used on a 24 million pixel full-frame DSLR, the overall mix of results from a couple of hundred shots confirmed the Sigma 70-200mm is up to the task. Surprisingly few lenses are.
In short, the reviewer seemed happy with its performance on the Nikon D3X. Given that BJP is a reputable professional's magazine, I'm tempted to trust that conclusion.