I use this guy for spherical virtual tours. Sigma is a great brand and has been coming out with some awesome third party glass that can be grabbed for a bargain. Although third party manufacturers have long been known for dull and lifeless images, Sigma has recently come under new management and changed their focus to pure quality. Sigma's new "Art Lenses" compete with or in some cases outperform those by the major camera makers. Just be aware that older versions from these brands in some cases don't have the consistency in sharp copies and quality control that match that of Canon, Nikon, and Sony.
The Sigma 15mm f/2.8 EX Diagonal Fisheye is a full frame lens that rivals those by Canon & Nikon. Due to their extreme effect, ultra wide angle fisheye have a maximum amount depth of field and allow creative and unusual framing.
Build & Mechanics
Sigma's fisheye feels very well built and substantial in the hand. The focus ring is very smooth and and pleasant to use, and with the Nikon version I'm pleased with the aperture ring. The aperture ring does indeed feature a lock (seen below) so that you can keep it at f/22 and ignore it allowing the camera to make aperture changes. Although this lens is not weather proof it feels like it is very durable and should hold up to some use.
The Sigma 15mm f/2.8 EX Diagonal Fisheye is available in Nikon, Canon, Pentax, and Sigma lens mounts but can also easily be adapted to Sony E-mount with a Metabones adapter. Since you cannot place filters over the protruding front element Sigma has included a filter slot on the rear by the lens mount.
Lens Resolution (Sharpness)
The Sigma 15mm f/2.8 EX Diagonal Fisheye is a little soft at f/2.8 but quickly improves as you stop down. I am almost always shooting at F8 or above in which the Sigma is quite sharp even holding up well on 36MP. The corners are definitely soft at f/2.8 but become acceptable around f/5.6. Below are some sample crops at different apertures. Overall this is a sharp lens and won't limit you with anything you'd be using a fisheye for.
Compared To It's Competition
The Sigma lines up quite nicely against the Nikon and Canon full-frame fisheyes and also comes in at a lower price. The Sigma 15mm Fisheye is an excellent option that provides good contrast and sharpness. This is part due to the fact that the Sigma is a newer design than the Nikon 16mm f/2.8 AF-D Fisheye and Canon EF 15mm f/2.8 Fisheye.
I've also heard problems that some have had with the Canon version hood sliding off in ones bag and exposing the lens' front element. This one of the few reasons I went with the Sigma version. Other than that the quality seems to be on par with the better price leaning towards the Sigma. I don't think you'll be disappointed.