Ultra-wide lenses aren't everyday tools for most photographers, but for some disciplines they're indispensable. I tend to recommend zooms at this end of the coverage range, as they're more versatile tools, giving you the ability to capture a wider variety of subjects without changing lenses.
Ultra-wide field of view; Very bright aperture; Very sharp; Solid build; Dust and splash protection; Available for multiple camera systems
Some barrel distortion; Soft edges and corners at wide apertures; Dim corners; Doesn't support front filters