Amazing lens IQ with the Sigma 35, that is for sure. Miles better than the Canon L.
However, I tried (2) copies and both had focusing issues in artificial light that my 35L's have not. It would front focus significantly when used in some indoor lighting (not all). I was constantly having to switch from 0 MA outdoors to +9 indoors (sometimes) and other times 0MA was fine. Drove me a bit insane.
Other then that quirk, it was incredible. I hope to try again in the future, as I don't want to give up on that amazing lens... I just know others have had the same exact issue happen with them and their 5D3, so it may be with that body only, who knows... Waiting to see more info as time goes on before rebuying, again.