There are various cheap manual 8mm fisheye lenses out there. Samyang’s offering appears to be the best of the field and has the advantage that it can be purchased in the UK without going through the Ebay store. The thing’s full name is “Samyang 8mm f/3.5 Fisheye Lens AS IF MC CSII (Detachable Hood), CANON EOS” which is probably a reflection of how many versions samyang likes to produce of its lenses rather than anything else.
In terms of build quality the lens seems solid enough and much the same applies to optical quality. I have had some slight issues with the focusing ring not always staying in place. I don’t spend time taking picture of test charts so I’ve leave the pixel peeping conclusions to those who do. The lens seems pretty sharp mind. One optical issue worth mentioning is that the lens apparently uses a stereographic projection resulting in a less extreme fisheye effect than other fisheye lenses.
So is it useful to wikipedians? To an extent. When I purchased this lens I was expecting it to be like the Samyang 800mm. More of a toy than a useful lens but the 8mm is a serious bit of optical equipment. Where as the 800mm only produced images useful for showing the optical effects of a mirror lens I could actually see some of the 8mm being used in articles. For example this image would be usable if I wanted to write an article about the Barbican conservatory.
Against that rectilinear lenses are likely to be more useful in most cases and for much the same money you can buy a Samyang 14mm. Still if you already own a 14mm lens and want something wider still the samyang provides a cheap and fairly high quality way to access the fisheye world.
All my use of the lens has been on a full frame camera which produces a slightly cropped circle as the resulting image. Looking at what’s on commons suggests that the results on ASP-C sensors should be even better although you get a slightly reduced field of view.
Final conclusion; If you have already reached the limits of rectilinear linear lenses and you can afford its fairly reasonable price its a lens worth owning.