Great if you have a fairly wide lens with small threads (like the kit lens). Can't beat the price. However you will not be getting tack sharp, stunning photography. You will essentially just get the effect, with probably aberrations, potentially weird glares in the sun, and apparently vignetted and lower quality/out of focus edges (none of this is nearly as bad as it sounds, but all still present). I bet it's a really good bang for your buck, and if you can't afford a $350 Canon/Sigma fisheye, then go for it. Look at some of the posted examples, highlights some of the flaws (though very low res, to their advantage). They seem fairly in focus to me, just not as tack sharp as you are going to get with a good shutter on a good tripod in good light with a proper Canon at f/9, mirror lock-up, using a remote
(which are in focus from edge to edge and don't need to be cropped).
I would wait for the .2 though. The effect will be much more dramatic, and you can always slim down by zooming in.
Basically 18mm becomes 6.3mm on the kit lens with the .3 (you may have figured this out), zoomed all the way out. But you have to zoom in a bit I think, to crop out the circle properly (see examples). Probably more like 10-14mm, and wider on the .2 Opteka Fisheye.