Sigma makes nice lenses but in terms of quality, especially when getting into the so-called "pro" lenses, you really are better off with a Nikkor/Nikon....
Tamron, Tokina and Sigma all make lenses with a Nikon mount and some of them are quite decent quality for a lot less money than you pay for a Nikon lens, but there are individual differences between the various brands. Also it seems that sometimes one brand will do a better job with "x" mm lens than the others but not as good a job with "y" mm lens. The best thing to do is to read the reviews of the lenses in which you are interested, then ask to put the lenses on the camera (either your own or a store's model) to see for yourself what the angle of view is. If you're lucky maybe the store will let you use your own CF card to shoot a few images with each of the different lenses so that you can then take it home and run it through your computer to make comparisons.
OTB
It is a full-frame fisheye lens, which means it fills the entire rectangular image with a curvy, distorted image and sees 180° from corner to corner diagonally. If you can imagine cutting a rectangle out of the center of a round fisheye image then you get the idea. Much more useful than a circular fisheye lens which only makes a small circle in the middle of your image.
