I've got two, but as I balk at paying 700 quid for a lens, I went for budget options:
1. Zenitar 16mm/f2.8, ~£135 off Russian trader on ebay.
2. Samyang 8mm/f3.5, ~£200 off Polish trader on ebay.
Both are manual only, but I only use them for manual, long exposure work so it's no big deal for me. The Samyang is a really nice lens for the price, holds up well right to the edge of the FOV. It has a pretty good underground reputation in togging circles. The Zenitar is not all that fishy on an APS-C, being 16mm, more like a standard wide lens, a little soft too but nothing Photoshop won't fettle.
If you want the 180 degree circular effect, you will need the Sigma 4.5mm/f2.8 for the crop sensor, or an 8mm circular for 35mm.