If you're talking about the Rokinon 35mm lens, I have an opinion to offer.
I bought his lens recently after reading good reviews, and used it on a shoot this past weekend on my BMPC 4K. The results were poor. Looking at the footage after the first day's shooting, I was shocked at the haloes and lack of sharpness that characterized this lens's images. The Canon still lenses I used for other shots absolutely blew the Rokinon away.
I was shooting with the Rokinon at 5.6 and wider, which was required for the depth of field we wanted. Here's a grab of the results with the lens wide open (or nearly so):
While I greatly preferred using a proper cinema lens (with gears on the focusing ring and hard focusing stops and manual aperture), the poor quality compromised the entire production. Even at less-than-wide-open apertures, the sharpness was obviously inferior to the Canon still lenses.