I had the opportunity to play with the the Samyang/Rokinon 14mm f/2.8 lens today. When matched up with a full frame camera it gives an amazingly wide field of view (I think I may have my new desktop background).
I actually found it almost too wide. The lens is all manual. Manual aperture ring and manual focus. Build quality is fair. The lens housing and lens mount are solid, but there was some play in the aperture selector ring. I'd put the build quality on par with Sigma's.
Image quality was in the good to very good range. When you nail the focus the pictures are sharp and with good contrast. Not as sharp as my 24mm f/1.4 mkII, but certainly sharper than most zoom lenses I've used (particularly the 17-40 f/4 L). There is a distinct yellow cast to the images that you may find yourself correcting in post though. Also (as with every ultra wide angle lens I've used) there is distortion. A portrait lens this is not. There are lightroom profiles available to correct the distortion. I didn't bother to do that with the sample photos though.
Overall I would recommend this lens. Considering you can get it for right around $350, it's a bargain.