I use or have used both Zeiss and Samyang lenses. I should point out that I like manual focus lenses to begin with, so your mileage can and will vary.
The Samyang, IMO, are great value for the money. Extremely sharp glass for the most part, but the wide angles do show more distortion, I think, than some of the better brand name lenses. Still for the money, and especially for the smaller sensor SLRs, it is hard to beat them.
I like my one zeiss so much I just ordered a second one. I would normally agree that brand name lenses work best than third party lenses, but the single exception, to me, is Zeiss. Excellent optics, sharp image, low distortion, built like a tank, and totally seamless integration with my Nikon.
I would recommend the Samnyang on any smaller sensor camera - Rebel, D5100, etc. But on your full frame D800, look at the Zeiss. Maybe rent one first to see if you like it.
Good luck
Joe