First off, bear in mind that I purchased this Sigma fisheye to replace Nikon's 10.5mm f/2.8 fisheye. While I was happy with the Nikon's optical performance, I was disappointed with the AF design. Nikon's lens relies on the AF motor in the camera, which makes for noisy AF. In addition, this design precludes full-time manual focus touch-up. And finally, the lack of an in-lens focus motor precludes AF on the existing D40/D40x and new D60, as well as future Nikon DSLRs lacking an in-camera focus motor.
In any event, when Sigma announced their fisheye lens a few months ago, I decided that I would purchase one. And I am very happy with it! The HSM provides autofocus performance as fast as the Nikon (at least on my Nikon D300 DSLR), while being eerily quiet.
As for the optical quality, it appears to be virtually identical to Nikon's, which is to say excellent. Both lenses are slightly soft wide-open (i.e. at f/2.8), but sharpen considerably when close down one f-stop. Indeed, both lenses are almost as sharp at f/4 as they are at f/5.6.
Besides the built-in autofocus motor, another difference between Nikon and Sigma's fisheye lenses is the field-of-view (FOV). Sigma's lens is 0.5mm wider, and provides a small -- but definitely noticeable -- increase in the angle of view.
So, it all comes down to this:
* Do you currently own a D40/D40x/D60, or plan to purchase one?
* Do you prefer super-quiet AF to the noisy mechanical variety?
* Do you prefer a better-built lens?
The one advantage that Nikon's lens has is size/weight. While arguably not as durable and well-built as the Sigma, it is lighter and more compact. If this is important to you, then you may prefer the Nikon. (See the comparison photos I posted to see what I mean.)
As for Cons, there is one. Sigma's lens is, unfortunately, not weather-sealed. I was hoping that Sigma would at least utilize a gasket around the lens mount, but that is not the case. As such, I recommend against using this lens in rain/drizzle -- or in a super-humid environment. (It is important to note that Nikon's fisheye lens is not weather-sealed either, and also lacks a lens mount gasket.)
Apart from the one aforementioned complaint, I highly recommend the Sigma 10mm fisheye lens. It provides all of the performance/quality of Nikon's fisheye lens, while providing fast and quiet autofocus to owners of ALL Nikon DSLR's, including the D40/D40x/D60. You will not be disappointed!