I have had this lens for more than a year so I know all the Pros and Cons; I did not just buy it and I am not writing a spur of the moment review. If you are debating between this lens and the Nikon one, I would go with this one. I am not saying this one in better than the Nikon because I haven't used the Nikon, but I don't imagine it is $350 better.
Now there are reviews that complain about the auto-focusing on this lens. It is a Macro lens meaning its field of focus is very small at 2.8 so it will tend to go back and forth at times, especially if there is a lot of things in the viewfinder. If you know how to use a DSLR then should be able to Manual Focus it. When shooting portrait style photos I have no problem with the autofocus, now with nature it doesn't always want to focus perfectly no, but like I said above it is a Macro lens and focuses very tight. Also a one person said to go to Manual Focus you have to flip the switch on the camera, one the lens, and move the focusing ring. Which is not true. The switch on the lens is a focus limiting switch. Which locks the camera in full minimum focus ratio, meaning it will focus closer up, or you can limit it.
I personally don't like the lens hood because it screws on, instead of twist and lock, which also means you can't put a filter on the lens directly while using the lens hood. Instead you have to put the filter on the hood itself.
Also it fits the Nikon but doesn't lock in the camera body as easy or smoothly, again it still fits and works fine.
I personally love my sigma lens and will definitely buy my next lens through Sigma. Most likely will be the 50mm Macro.