I’ve been using the Samyang (aka Rokinon, aka several other brand names) 14mm f/2.8 for several months now, off and on. 14mm isn’t a focal length that I can keep glued to my camera, but rather something I put on for specific uses and when I need a jolt to my creativity. 14mm is so different from 35mm or even 24mm that it gets my photographic juices going again when I look through the viewfinder. That reason alone almost justifies the low cost of this lens to me (assuming you don’t have the significantly better Nikon 14-24mm f/2.8 already); it also helps that the lens is sharp enough even at f/2.8 (in the center) to stand up to the demands of the D800E.
The biggest drawback of this lens is the huge distortion. It distorts like crazy. There are profiles available in Lightroom and Camera RAW, and probably in other applications like DxOMark, but distortion correction always involves tradeoffs (losing the edges, loss of resolution in the corrected areas). As a result you should carefully consider what you want to use this lens for, exactly. If you use it like I do – a lens to pull out when you want to get fancy/creative and do interesting things with perspective using the ultra-wide view – then it will work great. If you need it for professional architecture work or any other type of photography that requires accurate, straight lines (while maintaining a high level of resolution/image quality) then the Nikon 14-24mm f/2.8 (for Nikon shooters and even Canon shooters with an adapter), despite its high cost, is a much better choice.
Note that the Samyang 14mm f/2.8, like all the Samyang lenses released so far, is manual focus only. If you purchase the Nikon version that has an electronic chip, you can set it to the smallest aperture using the manual aperture ring and then control the aperture from the camera in aperture priority.
This lens also vignettes quite heavily at f/2.8, and is still noticeable at f/5.6. It’s another drawback of the design – keeping costs down while providing plenty of sharpness. As with distortion this is correctable in post with a lens profile, and it can actually be useful creatively. For me, vignetting is less of a concern than distortion because, well, I actually like how it looks sometimes, and it can serve as a kind of “natural” ND filter for bright skies.
You don’t usually associate ultra wide angle lenses with bokeh (rendering of out of focus elements), but thanks to a short minimum focus distance and the sharpness in the center of the lens at f/2.8 you can definitely use the Samyang 14mm f/2.8 to produce some interesting isolation of the subject matter. The clichéd “autumn leaf” shot (of which I am guilty of taking many) can suddenly get a lot more interesting when you use the 14mm to show much of the rest of the tree around that leaf, while still only maintaining the subject leaf in perfect focus. This is actually one of the main uses of a fast ultra-wide prime like this: getting up close but still showing the surrounding environment for context.
As a pure landscape lens , you could probably do better than this 14mm. For one thing, 14mm tends to be too wide for many landscapes, leaving the main subject too small in the final image (that said, it can definitely work with the right location and concept, and can help you get the foreground in a given shot to compliment the background). The abovementioned distortion is also so drastic that you will see it on most shots that have a smooth, visible horizon (think: ocean, flat lands, etc). Again, the Nikon 14-24mm f/2.8 is a much more flexible and highly corrected choice for landscape shooters (albeit significantly more expensive); other prime options that might work better include the Zeiss 21mm f/2.8 and the Nikon 24mm f/1.4 (again, these are much more expensive), or even going to the Nikon 28mm f/1.8 (I haven’t used that lens but have generally heard good things).
In short, the Samyang/Rokinon 14mm f/2.8 is an inexpensive way to get into ultra-wide angle shooting, and it is surprisingly sharp (at least my copy is) for the price. It suffers from huge levels of distortion (the lack of distortion correction goes hand in hand with both the high sharpness and the low cost – you can’t realistically have all three, it’s a “pick two” kind of situation), and it is manual focus only, with decent but not professional or Zeiss level build quality. It isn’t what I would consider a “magical” lens, but the low cost makes it easy to stomach if you want to have 14mm in your arsenal. As your primary landscape or architecture lens you could do much better, even if you have to save your pennies for a while; but as a means to give your creative juices a jolt and have a little ultra-wide fun (if you don’t have a different 14mm already), then this is a good purchase.