Summary: I do photography and video for weddings. I just upgraded from cropped to Full Frame (T3i to Mark II) and I've had a chance to use this for both photography and video on both cameras. I love this lens, however, it's probably not for everyone. It's much darker in the corners on a full frame camera and at times this lens can feel like a fisheye (which can be good or bad). I use this on a steadicam with my t3i and it looks pretty good.
Summary: I have high hope for this lens due to the so many positive reviews. OTOH, I don't have much expectations because it is so much cheaper than comparable Canon, Nikon or Pentax counter-part. Let me just get to the bottom. It is extremely sharp and has incredible resolution. It is perfectly usable wide open at f/2.8. Color and contrast are great. Other than distortion, it is optically perfect. Distortion can be easily corrected with software such as PTLens.
Summary: Absolutely perfect for my needs. On Pentax cameras, Samyang lenses (and their products sold under different brands including Rokinon) give you an A setting on the aperture ring. I grew up in the manual focus era so that's no problem. The lens hood is fixed and adds girth. I heard complaints about the lens cap. I see no problem. The pouch does seem useless but then, I don't use lens-specific bags or cases anyway (I put everything in a padded backpack).
Summary: ordered it, got the sony-mount instead, sent it back and got another sony mount lens. ended up returning it for refund, very disappointed EDIT: Now that i finally recieved the lens (4th times the charm right?) I've got to say, the hassle was well worth it. Slight distortion in the lens that adds to the whole wide-angle vibe, ungodly sharpness edge to edge, awesome angle of view and an amazingly short minimum focusing distance.
Summary: I was really reluctant about this lens since I have the Pentax DA 15mm limited and I had some bad experiences with Korean made lens. What a surprise! This is a good surprise. Positive points: 1. very affordable. 2. full frame capable. 3. very well built (except for the cap). 4. feel sturdy in hand but relatively light. 5. long and smooth throw for manual focusing, what a treat. (while not Pentax (K, M, A) smooth, good enough) 6. resolution high even at f2.8. 7.