I recently purchased two new, off-brand, lenses for use with my Nikon D300. I needed something faster and better built than kit lenses, especially now that I am photographing so many weddings and events. My choices were a Tamron 17-50mm 2.8 for a standard zoom and a Sigma 50-150mm 2.8 for the tele-zoom. I did a lot of research prior to making the decisions and am very happy with the image quality of both lenses, even at f2.8. The Tamron AF does hunt a little in low light but more often than not it finds its mark and is tack sharp! The Sigma is just as sharp and does not seem to have any difficulty finding focus.
I mulled over the Nikon versus off-brand issue for far too long and hope this post helps others make a quicker decision to go off-brand. If you have the cash then, by all means, buy the Nikon lenses. The Tamron was a mere $400 and the Sigma $675. Compared to Nikon’s equivalents at $1400 and $1800, Tamron and Sigma were easy choices for me.