This lens is what I consider a beast! Prone to flare, but typical for fish eyes. The particular one I got was the Rokinon one (Samyang manufactures them for about 5 different brands, with extremely slight differences between them). The Rokinon runs about $290, the Vivitar one (Which I will be purchasing in about 2 weeks) is about $330, the Opteka one runs about $270, the bower one is around $270 also I think, and the Samyang labeled one is about $350.
They are all essentially the same, with only a minimal difference in the range (7mm, 8mm, etc..), the focusing grips seem to be different also, and the letter coloring. I originally wanted the Vivitar one but my source was out of them at the time, so I picked up the Rokinon, and honestly, couldn't be happier - best $290 I have spent on a budget lens purchase.
The ONLY issue I have with it (the flare is sort of an issue too, but most of the time good pp can get rid of most of it) is the lens hood, it is not removable, and that leads to the issue that that causes, and that is the ability to use it on full frame cameras, you can not. You can cut the pedals off of the lens hood and then you would be able to use it on full frames, but I am not quite ready to do that yet. So, earlier today I purchased a Takumar 18mm (baby) FE from the marketplace here so I can shoot some fish eye film pics.