Go tired of waiting and got a Tamron... only taken some shots, but based on that...
Soft @ f2.8, sharpens at f4
Vignetting at 2.8
No visible Onion rings so far
Friend of mine(pro-wedding 15yrs plus) bought and returned to BH for same reasons PLUS slow AF
Sometime I wondering if Canon & Nikon deliberately "tinker" with their firmware updates to make third party lenses not to auto focus quicker or introduce some other bug to basically not make them work as well as they should. I say this because two years ago I had Tamron 18-50 f/2.8 OS that used to work brilliantly with my EOS 60D but once I updated my firmware the lens became slower at auto focusing.
5D MK III, 16-35/2.8 L II, 24-70/2.8 VC, 24-105/4 L IS, 70-200/2.8 L IS II, 100/2.8 L IS, 40/2.8, 50/1.4, 85/1.8, 150-500 OS, 600EX-RTx2, ST-E3-RT, Rokinon 24/3.5 T-S, D7100+18-300 VR, YN568EX, 055CXPRO4+468MGRC2, 190CXPRO4+322RC2, Gorillapod Focus+496RC2, CamRanger, B+W filters, Konova K3/31.5 etc