Some of the 3rd-party lenses are quite good, some aren't. The really good ones don't usually wind up being a whole lot less expensive than the still slightly better Nikon counterpart.
Another point of departure is warranties.
The one Sigma lens I have purchased, the APO 150-500 mm f/5.6-6.3 DG HSM did not have an equivelent in the Nikon line up. I have been very happy with the build quality and performance of the lens, considering it's price.
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Keith Harrod | Image Works
Adobe Bogen Dell Giottos hdrSoft Imagenomic Lexar Nikon Sekonic Sigma Topaz Labs Vivitar Vagabond Wacom Westcott Western Digital
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