I recieved my 8mm fisheye lens yesterday and really is a great lens. I bought the Samyang 8mm which is rebranded in other countries as different names like Pro-Optic, Samyang, Rokinon, Bower etc. The construction is great, feels very solid with a metal touch about it rather then the cheap plastic on the lessor Nikkor lenses.
Having a fisheye really makes you go for shots that you would not take with any other lenses, I feel like I need to get in really close and low. I know fisheyes can be a love or hate affair, but you take shots you would not normally take so I just see it as a tool to get more variation.