I think the Sigma 28-300mm is a scam. Here's why, I have racked the Sigma out to 300mm and taken a picture and then taken my Nikon 70-300mm and racked it all the way out and taken the same picture and noticed the the Sigma doesn't get me near as close as the Nikon. Which it should because they are both 300mm lenses(on a 35mm camera). As far as the performance, to be honest my 18-55mm kit lens is sharper than the Sigma at the lower end and it is just as soft as the 70-300mm Nikon at the upper end. The only difference is that the Sigma is slower at the top end then the Nikon. The AF is faster but that is about it. The only time I really use it is when I am going to be taking pictures of stuff that doesn't really matter and I don't want to carry more than one lens. For instance when I am taking snapshots of friends at the gun range. But other than that it sits around. And now that I am getting the 50mm 1.8 and the 70-300mm VR I probably won't ever use it.
As for the difference between Nikon, Sigma, and Tameron, I am still to new to give a really experienced answer. I can tell you this though, I have spoken to experienced people and they have told me that there are Sigma lenses that will blow away Nikon lenses and there are Tamaron lenses that will blow them both away. On that same note there are lenses from Nikon that are better than both, and so on and so on. It really depends on the specific lens you are looking at by a specific manufacturer.
