I currently use the Sigma lenses and most of my photos on this site I used the Sigma products mentioned but previously on the D30 I used a tamron lenses I prefer the latter but presently spending my money elsewhere.
But for rough and tumble type areas the Sigma have taken a beating and still provide a reasonable nice photo(s).
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Regional Coordinator for Operation Lifesaver
Southern Nevada and Southern Utah
A lot of people like Sigma.... I recently started using a Sigma 28-300 (I started about a week and a half ago). The quality is right where I'd like it. Many think Sigma is better than Tamron and vice versa.... Kinda like the endless Canon vs: Nikon debates. Personal preference.
All my photos currently on RP.NET are shot with my Sigma 28-105mm and my sigma 70-300mm. Im happy with both lenses and have no problems at all with Sigma lenses.