The Zeiss 18mm f/3.5 is a superb wide-angle prime lens. It's not the fastest lens, and it doesn't zoom, and it doesn't autofocus - but if you want great, great image quality and you can live with those limitations, then this lens won't let you down.
The build quality is simply superb - far better than anything in Canon's wide-angle range. The lens is absolutely rock solid, nothing flexes or rattles, and the focus ring is as smooth as you could want.
Image quality is superb too. Corner-to-corner sharpness is excellent. Flare and ghosting is very well controlled. It does suffer from some chromatic aberration, but that's easily fixed in post-processing, whereas flare isn't. With the possible exception of the TS-E 17mm f/4 L, this is the sharpest ultra-wide-angle lens available for Canon DSLRs. (But the TS-E costs a lot more and doesn't accept filters.) So it's a great lens for the serious landscape photographer. It also features a hard infinity stop on the focus ring and an accurate infinity focus, which means it's particularly good for astrophotography and photographing the Aurora Borealis.