My original Nikon full-frame fisheye was a MF f3.5 version that I lost whilst shooting in a waist-high meadow full of natural grass. I used it on my film SLR (F100). The insurance company's replacement was this one, that I now use on my D700.
I still would , now, have preferred to have my older, slower and bought secondhand original Nikkor. Optically, this is fine though comparisons with other lens are impossible as there's only two available and one simply shouldn't need to buy a fisheye lens again.
It does have excellent multicoating, essential for coping with a bright sun that's nearly always in the frame. A fireball of sun in one corner doesn't result in a string of coloured globules raking across the frame. Centre sharpness is excellent, with the edges and corners are good and OK respectively. Colours and contrast are bright and punchy.
The plastic lens casing is marked as recyclable - don't get me wrong, I'm not against the environment but the casing does seem, certainly next to traditional Nikon values as though it won't last long. I dropped this lens onto a concrete floor from only a couple of feet, which dented the shallow hood and fractured some of that plastic casing, but it still works fine, so the basics are durable. A warning there; it's so small it can easily slip though cold, especially gloved, fingers.
In urban landscapes everyday images can be transformed into a breathtaking and winning image. Environmental portraits showing the subject's surroundings around them can be an inspired alternative. Use care, however, as close-ups with this lens can cause enemies. Pop and rock bands, with their lust for 'awesome' images of themselves love me using this lens occasionally, either at gigs or on a shoot. More traditional photographers, however, often see it as gimmicky, though.
Instinct might tell me to go for the Sigma, had I not got this one. Optically, I'd stay with this but for once, I'd say that the Sigma might be better built. I haven't handled or used their's, so don't take my word for it. Maybe, my dropping the lens and subsequent damage blinkers me somewhat.