Samyang have a number of relatively-affordable ultra-wide-angle lenses. This review is about the 14mm f/2.8 intended for full-frame cameras, but much of it applies equally to the equivalent 10mm f/2.8 for DX / APS-C cameras. Performance of the two lenses is similar according to reviews I've read.

Side note: what is "normal", "wide-angle" and "ultra-wide-angle"

"Normal" or "Standard" lenses have an angle of view of 40-60º, which tends to have a natural appearance with many subjects. This corresponds to a focal length of 35-60mm for full frame / 35mm cameras, or 25-40mm for DX / APS-C cameras. A "normal" lens gives approximately the same sense of perspective as the human eye.  "Wide-angle" is typically to describe a focal length of 24-35mm for full-frame, or 16-25mm for DX / APS-C. Wide-angle lenses are good for architecture and landscapes, but tend to make pictures of people look unnatural, as the photographer has to be rather close to the subject. Typical zooms lenses bundled with DSLR cameras cover the range of focal length from wide-angle through "normal", usually to medium long focus (i.e. greater focal length than "normal").

Ultra-wide-angle is anything below about 24mm for full-frame or 15mm for DX / APS-C. These lenses have an enormously wide angle of view - perhaps 115º - which is great for some subjects such as landscapes or interiors of buildings, but can be unnatural and even disturbing for other subjects. For example, an ultra-wide does not make a great portrait lens:

These images above are taken with a full-frame camera for which an 85mm or perhaps 50mm lens might be appropriate for portraits.

The two Samyang lenses reviewed here typically cost £270-300 (the Nikon fit 10mm is just over £300), which is a tiny fraction of the price of most ultra-wides. The sharpness and chromatic aberration (CA) are very good at the centre, and fairly good at the edge (better when stopped down a bit). To quote the photozone review (listed at the bottom): "the Samyang produced nothing short of outstanding resolution figures for a lens in this class".  For both sharpness and CA they are as good as some much more expensive fixed-focus ultra-wides, and better than most ultra-wide zooms. However there are trade-offs:

1. Manual focus only. This is not a severe problem, as ultra-wides have a large depth-of-field, so focus often isn't critical except for very close subjects.
2. Fixed focal length. You expect a good quality zoom at this price?
3. Significant geometric (barrel) distortion and vignetting (the latter mainly at wide aperture). Surprisingly, even these aren't as serious as one might think. Many post processing software packages (including Lightroom and Photoshop) include lens correction profiles that can correct the distortion and vignetting almost completely.
4. Some reviewers have reported getting "bad copies" of these lenses. Perhaps the quality control isn't perfect for Samyang lenses; always best to take and check some images after purchasing any lens, and return a lens that isn't up to snuff.

Here's an example showing the distortion, and the same image with the lens correction profile applied:

Out of the camera, no correction applied

With Lightroom lens correction profile applied

The Lightroom lens correction profile corrects both the barrel distortion and the vignetting (dark corners of the uncorrected image). Notice that the correction loses very little at the edges. With some lenses, the correction distorts the shape so that it has to be cropped, losing significant areas around the edge.

Out of the camera, no correction applied

With Lightroom lens correction profile applied

Even uncorrected, the goemetric distortion is hardly noticeable unless there are lines parallel to the edges. The sloping verticals left and right in these images of Market Hill are perspective distortion (not lens distortion) as I was pointing the camera down towards the car rear light.

These Samyang lenses have such a wide angle of view that they can be tricky to use in sunlight: point one way and the sun is in the image, point the other way and the photographer's shadow is in the image! Yep, that's my shadow bottom-right in the Market Hill shots above.

Here's one into the light:

In this one the sun was in the picture until I cropped some from the top.

Landscapes can also benefit from ultra-wides: I managed to see the Northern Lights recently, and they filled the whole sky:

Here are some reviews that I found useful, first for the 14mm:

http://www.northlight-images.co.uk/reviews/lenses/samyang14.html

http://www.photozone.de/canon_eos_ff/532-samyang14f28eosff

https://www.ephotozine.com/article/samyang-14mm-f-2-8-ed-as-if-umc-lens-review-19621

https://www.imaging-resource.com/lenses/samyangrokinon/14mm-f2.8-if-ed-umc/review/

Ken Rockwell also wrote a review that suggested to me that he hadn't actually used the lens, as some of his comments were nonsense.

For the 10mm lens:

https://www.ephotozine.com/article/samyang-10mm-f-2-8-ed-as-ncs-cs-lens-review-25473

http://www.lenstip.com/407.1-Lens_review-Samyang_10_mm_f_2.8_ED_AS_NCS_CS.html

http://www.thephoblographer.com/2014/04/20/review-rokinon-10mm-f2-8-pentax-k/#.Vva8mfhxo-V

The 10mm lens has very slightly more barrel distortion, but has a more modern anti-flare coating.

Overall: good lenses, especially if you use post-processing software that has lens correction profiles. Very good value.