[dpvideo]

I will give you a choice. You can pick Samyang Fisheye for an incredibly high performance and a totally affordable price. Or, You can go ahead and spend 3 times as much of your precious money on Sigma or at least 4 times as much on Canon. For the quality of its optics and build quality, Samyang is the best bargain. I mean, after all, it's your choice, but I would say go with Samyang. Also, Samyang is the mother brand for Rokinon, so you might as well go with Samyang. If you are still struggle, you can always find what amazing things they said about Samyang here on Photozone or on any other forum. Samyang's got its own facebook page, and they are also holding this big photo/video contest. If you choose to go for Samyang, you might want to look into it.
https://www.facebook.com/SAMYANGOPTICS
 
As far as I know, you can get the best deal for Samyang at B&H or Adorama.