The Sigma is an acceptable performer and it's AF but should focused manually due to the field of view. Build quality is quite good. When compared to the Nikon fisheye the Sigma doesn't map as accurately and it suffers from higher levels of chromatic abberation. However given the Sigma is roughly 1/2 the cost of the Nikkor it's a fair trade off for non critical professional use.
I've found the cheaper Russian made Zenitar fisheye to be a better performer overall than the Sigma, however it's MF only and the build quality varies considerably from one lens to the next.