The 12-24mm FF lens has been a Sigma staple for a long time. This is a re-design, hopefully better than the previous ones. It's a good time to be an Ultra-Wide Angle lens DSLR photographer - Irix 11mm coming up, Irix 15mm f/2.8, Laowa (?) 12mm f2.8, Samyang 12 mm f/2.8 fisheye and slightly redesigned 14mm f/2.8, Tamron/Pentax 15-30 f/2.8, Sigma 20mm f/1.4 (!), not to mention the Canon 11-24, 16-35 f/4 IS, 17mm f/4 TS, and Nikon offerings (with which I am least familiar) newer than 14-24 f/2.8. Plus, for macro lovers, the Laowa 15mm f/whatever 1:1 macro, with addied shift function for APS-C/ DX users.
I find it quite difficult to use ultra-wide angle lenses effectively. It is harder (for me) to come up with a composition that works and doesn't look either boring or gimmicky.