I just received my Rokinon 8mm fisheye lens today, and I'll work on comparing this lens to my current fisheye lens, the Pentax 10-17mm. I know many people are considering getting a fisheye lens but don't want to spend too much money on such a novelty lens, so there's a lot of interest in the 8mm fisheye, especially compared to Pentax's own 10-17. I haven't seen a direct comparison between the 2, maybe because no one actually owns both lenses? Anyway, I'll compare these two lenses, and eventually I will keep only one of them. I think it will be the 8mm though, because I rarely ever use the zoom function on my 10-17!
I don't have a Pentax DSLR with me at the moment, so the actual image comparison on APS-C will have to wait, although I can do some preliminary tests on M4/3 with my adapters.
For now, however, I'll write briefly on the physical attributes of the 2 lenses. They both weigh about the same and are around the same diameter at the barrel. However, the Rokinon is wider at the front lens element. Both lenses are made of plastic and metal, but the plastic on the Pentax feels just slightly better. The Pentax's hood is made of metal but the Rokinon's is plastic. Same with the hood - Pentax's slip on cap is made of metal and is held on by friction while Rokinon's is made of plastic and has a pinch clip.
The focusing ring on the Rokinon is really well-damped and is a joy to manual focus, although I don't think there's much manual focusing that needs to be done with this lens. The Pentax has quick-shift, although that makes it very loose and easily bumped, especially when I'm shooting with it on my M4/3 in manual focus. In this regard, I personally prefer the Rokinon's manual focus and its smooth focusing ring.
One thing I noted is the Rokinon's focus ring markings are way off compared to the actual focus. When I'm focusing at something about 2m away, the lens must be set to about 0.5m. Turns out this is a common issue because they don't seem to calibrate the focus for the individual lens mounts so most lenses are off. The fix is easy enough (loosen some screws under the focus ring rubber and calibrate it to focus at infinity) but it's also possible to just set the lens to 1m and f/8 and shoot since the DOF will be more than sufficient for fisheye shots.
Here are some photos of the 2 lenses: