To be honest, and if dpr's samples are anything to go by (they mostly aren't), nope. Images look kind of sharp all over... kind of. Things change a lot with some USM, which in itself makes me skeptical since that means the lens isn't really that sharp to start with, and even with sharpening, it seems to lack "life".
When that is said, it probably does the job that it's supposed to do, but so do many other 14-15mm lenses. The large aperture is a bonus of course, but only for those who need shallow depth of field at this wide angle and/or do a lot of ultra-wide, low light photography using a camera without stabilisation (like with a Nikon
).
Ultra wide-angle has become almost mainstream the last decade, but most of the images I see taken with these lenses look weird and distorted rather than interesting and beautiful. Again, it's an important tool for some, but I would much rather pay this kind of money or more for a great, much more usable 21mm than for this tool that has rather limited use.
If I'm going back into full frame, it's partly because I miss the Zeiss 21mm so much. Kind of good ultra-wide lenses are available for most formats.
Edit: An ultra-WA lens that I would love to try though, is the Laowa 15mm f/4 macro. It focuses down to 12.2 cm (1:1), weighs a third (410 vs. 1,170g) and costs a third ($499 vs. 1,599) of the Sigma
and it includes 4mm shift
and it takes ordinary filters!