Roger at Lensrentals.com has put the brand new Sigma 14mm f/1.8 DG HSM Art through OLAF testing and compared it to various lenses of the same focal length, most notably the Canon EF 14mm f/2.8L II.
From Roger:
Let’s face it, there hasn’t been an impressive 14mm prime for SLRs for a while, and I haven’t heard the natives banging on the castle doors demanding one. But for some people this is going to be a really fun lens. I consider it reasonably priced for what it is; a unique and excellent lens that hasn’t been made before. I shot real-estate for a brief moment in time, and I would have killed for this lens then. And I like playing with ultra-wide images in general, so I’m kind of excited about it. It won’t be a huge fraction of my images, but it will be a tool I didn’t really have before.
A lot of people will never shoot 14mm and will never notice this lens exists. But for those who do, the Sigma 14mm f/1.8 Art Lens is probably going to be a fascinating lens indeed. Read the full review
It shouldn't come as a surprise that the Sigma 14mm f/1.8 DG HSM Art is better than the much older Canon EF 14mm f/2.8L II as far as optical performance goes.
Sigma 14mm f/1.8 DG HSM Art: B&H Photo | MPEX | Park Cameras (UK)