With all the positives about the Tokina 10-17m f/3.5-4.5 it is probably not surprising that there hasn't been more interest in the Sigma 10mm f/2.8 EX DC HSM Fisheye lens. When comparing the two lenses the obvious advantage the Tokina has is that it is more versatile with its zoom range. The Tokina may also be about $50-90 cheaper. But for all the other following reasons the Sigma is my choice:
>Simpler - No port extensions, no zoom gear . no diopters
>Image quality - comparing the f2.8 to the Tokina's 3.5 - 4.5 you would expect better image quality from the Sigma. I think they are both excellent in image quality but in terms of underwater use I found the corners definitely sharper with the Sigma ( Now I have to admit that when I used the Tokina I did not have an extension ring but looking at other uncropped images from the Tokina it seems to be true generally ).
{I also think that it is generally agreed that Nikon 10.5mm fisheye does rather poorly with corner sharpness - an uw pro who uses the Nikon 10.5mm admitted to me that his images with this lens always need a bit of cropping.}
>Brighter viewfinder image
>Faster and more accurate autofocus. ( I can't proove this one but it certainly seemed that way for me )
>Even closer focusing
>Great for split shots. This is particularly true for the Canon version I think as the 167 degree view is a bit less fishy than the 180 degree Nikon mount.
Anyway just a thought. I have been very happy with mine