The Rokinon (= Samyang) 8.5mm is "good enough" for video and/or use in bright light. It is soft wide open, and therefore terrible if you want to shoot photos in available light.
If you are shooting in the snow, you will stop down your lens anyway, so the Samyang is absolutely fine. Its build quality is also second to none.
To get an excellent (= sharp and bright) fish eye I consider two options:
- Nikon 10.5mm f/2.8
- Pick up a cheap micro four thirds body and get the Samyang 7.5mm f/3.5 Fisheye (MFT)
Both options should run you the same combined total price (MFT bodies can be found for around $300-350). For some reason the MFT version of the Samyang is tack sharp even wide open. In my experience it is nearly as good as the Nikon. It is orders of magnitudes better than the Canon or Nikon version.
Anyway for me part of the fun of a fish eye is taking photos at parties... and for that nothing beats the tack sharp f/2.8 of the Nikon.