I have a Nikon 10.5mm, Sigma 8mm EX DG and Nikon 8mm/f2.8 AIS (all used on a Nikon D200).
The Nikon 8mm is one of Nikon's legendary lenses, far superior over the Sigma 8mm. Truly the best for handhold action panoramas involving many people. The most prominent user of this lens is Jook Leung (http://www.360vr.com/
Buyers beware: The older AI version has a different coating and is prone to flare, which is a showstopper for VR purposes. So e.g. do not buy this one:http://cgi.ebay.fr/ws/eBayISAPI.dll?ViewItem&item=330020906332
If you want, I can provide you with test images of all three lenses. Just tell me what subject would be best for your purposes (I guess architectural with a lot of straight lines?). I can also provide different sequences, e.g. 4+2 (4 around, 1 up, 1 down) and 3 (60° oblique technique) for the circular fisheyes.
I am really looking forward having fisheye support in APP. With that and all the other improvements planned you will have a real winner at hand. (Just as a sidenote: Realviz Stitcher just introduced fisheye support and failed miserably for the 10.5mm, so there is a lot of opportunities for the competition).