Upload & Sell: On
I don't have a comparison handy (and will likely not get a chance to test it, but I've used the Zeiss Planar enough to give you my thoughts:
Wide open: No contest...the Rokinon walks all over the Zeiss in sharpness, is equal or better in bokeh, and pretty close in color. Zeiss has a bit better contrast.
@ f/2: Pretty darn close in sharpness, though I might give a slight edge to the Rokinon. Bokeh is similar. Zeiss better in color and contrast, though only just.
f/2.8 and smaller: Zeiss is sharper, becoming critically sharp at f/4, with better microcontrast and a little better color. Bokeh is similar. However, the Rokinon is no slouch here, and while not quite as sharp as the Zeiss, is still very sharp...certainly nothing to complain about. However, this is really where the Zeiss shines. Beautiful 3D rendering that is hard to match.
I knew the Rokinon/Samyang and its clones were great optically from the reviews and threads here, but I didn't realize how solidly built the lens would be. Metal focus ring and main barrel, high quality plastics for the aperture ring and the body close to the front element. Great focus damping with absolutely no slop at all. The only build thing to complain about is that the hood feels cheap, and doesn't fit as snugly as I might like, and the lens cap that comes with it is a cheapo cap. But the lens itself is extremely well made for a $250 lens, and feels like the best lenses from Canon or Nikon in build.
Oh, and just as an aside, in case anyone was wondering about the beautiful bokeh and razor sharpness of the shot OF the lens itself...that was taken with my new 100 f/2.8L IS, which might be the most impressive lens optically I've ever used, all things considered.