This ultra-wide 14mm prime lens is available for many different camera mounts (Nikon, Canon, Pentax, etc), and covers a full-frame 35mm sensor. Compared to Canon's 14mm f/2.8 lens, The Samyang is much sharper (especially in the corners) and dramatically cheaper (at the time of writing it is less than 1/7th the price!). It also has much less chromatic aberration. This is a fantastic deal for any full-frame shooter.
Its fast f/2.8 aperture combined with its wide focal length allows photos to be taken in ultra-low light without suffering from camera shake. It even allows a little bit of background blur if the subject is very close to the lens.
This lens has a fixed hood which protects its bulbous front-element, and the lens cap is also gigantic. It certainly takes up a lot of space in the bag! One build issue that is a little annoying is that the focus scale is particularly badly calibrated out of the factory, so when you're focused to infinity, the focus scale might only be reading "2 metres"! This is irritating because the focus scale would normally be an excellent way of manually focusing the lens, given its huge depth of field. On my lens, I have to remember that infinity occurs just before the "3 metre" mark.
Despite the large front element, it is quite flare resistant, and I don't hesitate to include the sun in the frame. It creates pleasant "sun-stars" when stopped down.
Its only real optical disavantage is its ridiculously high distortion. For scenes that involve a lot of straight lines (like architecture shots), it's always best to correct this distortion using a Lightroom lens correction profile or other correction software. After correction, very little distortion remains.
Here's a before-and-after comparison of distortion correction. Hover over the labels underneath to switch between the corrected and uncorrected versions, or click on the image to view full screen.
Distortion correction sample
My version is the Nikon F mount one, so I adapt it to the Canon EF mount using a Fotodiox Pro adapter. This adapter has an "AF-confirm chip" built in which provides EXIF data (focal length and maximum aperture), and enables the camera's "AF confirmation" light that lets you know when the image is in focus. However the combination of this lens being so wide, and an unusually long focus throw on the focus ring, means that there is a large area of the focus ring that will still show as "in-focus" according to the camera's AF sensors, making the AF-confirm feature not useful for this lens. For most shots, I focus using the distance scale. For close-ups, I focus using liveview's zoom feature.