Hey there! I'm a DP, own a 7D, 5D mkII, and a RED Scarlet.
I have many lenses, but right now, I mainly use my Rokinon case. Even on the Scarlet.
If you can wait, in september they'll have all of the lenses out with the cine version, with follow focus gears on the focus ring and aperture ring, declicke aperture, and marked as T-stops instead of F-stops.
They're an awesome set, I've shot head to head with Carl Zeiss (they're better but only noticeable with tests), Leica (didn't like the look on those), Canon L (they're just as good as Rokinon, sometimes they don't get to that point, but they suck at manual focusing because of the short travel between minimum focus distance and infinite).
I'm shooting commercials for Samsung, Coca-Cola, and brands alike. Everyone loves the image it renders.
Go ahead with Samyang/Rokinon. You won't regret it.
About this specific model, at f1.4 is very soft. Very. Maybe I got a bad copy, dunno, but I try not to use it at that point. But I'm refering to pro work where I must show an incredible level of sharpness... I'm very picky with that. With work that isn't that important, f1.4 rocks.