I have and love the Samyang 8mm fisheye. (I've got the Rokinon branded one and shoot on a Canon 50D). It has outstanding sharpness, color reproduction and contrast. I too got the lens after seeing what it could do on this board.
It's designed for crop bodies so you get a real fisheye effect on a crop body, not like the Canon fisheye that's designed for 35mm/FF format where you just get an UWA effect because you only get the center of the image. If you put this lens on a FF camera (some brands you can) you will get the circle fisheye with black around it.
But here's the catch with this lens... it's all manual (exposure and focus). You have to work for that image so make sure you're good with that. And on the Canon version there is no focus confirm or useful TTL metering information. I use my hand held meter or take a few shots and go off the histogram to adjust my exposure. But don't let that scare you off... it's not difficult to learn to use it well and it's the most fun you'll have with an under $300 lens.