I have the Sigma Art 50, 85 and 135. Had screwed around with the Sigma USB dock when using them on the 6DII, but set everything back to zeroes to use them adapted on the R, and I haven't touched the dock since. They all focus fast and spot on. The 85 Art with the R's updated eye focusing is outstanding.
For the EOS R it'll be interesting to see what Sigma does. I can't imagine the Sigma RF mount would have the function ring like Canon? Personally I'd probably only use a Canon zoom. As for prime lenses I expect any Sigma Art RF will be optically excellent so it will probably come down to a cost, performance, features, show down with the Canon equivalent.
I'm not a pro so I don't need the RF 50 1.2 for $2100, but the rumor here a while back was Canon will not be making an RF 50 1.4, just a 1.8? If Sigma makes a 50 1.4 I'd have to consider it. But if Canon's rumored RF 50 1.8 is anything like the RF 35 1.8 in features, size, IQ and price I'd definitely go with Canon