Good enough. I've tested a lot of options in the 10-12mm range, and they all come with liabilities. The Sigma's liabilities are more livable for a landscape shooter than the alternatives.
A good value. I've dragged this lens around the world. It holds up to my abuse and gives me that good enough performance I just mentioned. Considering it costs less than the Nikkor, that makes it a decent value, too.
Might have some sample variations, so check before making purchase