I've got the Sigma 15mm and 8mm fisheyes and use them regularly on my D800.
Just like all fisheye lenses (even my Nikkor 10.5mm and old 16mm 2.8 AI-s and the old NPS 8mm 2.8 from film days), you'll get fringing. With ACR, it's an easy fix and the current LR and ACR it's automatic if you check the box.
I considered getting the current Nikon 16mm a couple of years ago after going full-frame and lusted for the old 8mm 2.8, but after reading the experiences with the Sigma at Lensrentals.com, decided to save the money and got mine on eBay for about 1/2. I was happy using it on my D700, D3 and now on the D800 and the image quality is just fine.
For one of my clients, a client who is an old-world Orthodox iconographer requires the circular-view for his portfolio since curves, domes and arches are part of the Byzantine tradition. I don't use them every day, but when I do, nothing else will give you that nice curvilinear rendition and nothing but a circular fisheye can show the entire interior from floor to ceiling and 360-degrees in one photo.
Not all photos will benefit from the distortion from any of these fisheye lenses, but there are many images that will work with a little work and imagination.
As to working with the Sigma 15mm, I can highly recommend this lens on your D800.