This was one of the least expensive fisheye lenses I could find that holds the capacity to produce great results... but because in photography the saying "you get what you pay for" truely lives there are a few drawbacks to this lens:
The first is that it is manual focus so it takes constant adjustments when you're in the higher f-stop range not to mention you have truly no idea if you're in focus or not because everything appears so far away, but in f/11 or higher you do not need to worry about focusing, just leave the focal distance at about 6ft and you're good to go.
The second is the f-stop ring. It is very hard to see at night and rotates very easily which can lead to poor photos if you're not careful.
The third is the lens does not communicate any information back to the camera such as the aperture values, so it takes a pen and paper (or photographic memory) to figure out what the aperture value for a particular shot was. This is why many of my fisheye photos have an aperture value of "N/A."
Fourth is the chromatic abberations are on the higher side, but this is an issue for all fisheye lenses due to physical disadvantages that are not very easily overcome. After processing a RAW image with Canon's bundled software the abberations are quickly reduced and frankly hard to notice. I would not reccomend this lens to be used in harsh lighting unless you're shooting in a RAW format.
Lastly this is meant for crop sensor cameras so it is not meant for 35mm film or full frame DSLR's. The lens does fit EF mount bodies so with some modifications it can be used as a circular fisheye.
Overall, though, I will have to say it was worth saving the money and not buying the Sigma autofocus version because after a few outings and discussing our problems over a bit of wine and some sushi I've come to love this lens very much. Once you get used to how it functions it can become a very favorable lens in your bag and at only about $360 it is very reasonably priced.