No regrets moving from a 1Dx (aside from having no built in grip). Having come from a 5Dc, I can say the R is another type of dream. Larger file sizes are making my machine struggle a bit (even with 32GB of RAM), but, shy of that, getting more and more used to using the body. Adapted glass, as many have mentioned, works just fine. Beyond that, it's your comfort level with ML cam's. They are the future, no matter what anyone says.
As some have mentioned, prior, pair it with a SDXC UHS-ii card with v90, as the minimum standard, and the camera will react very nicely. Benchmarked to be one of the best, with the R, is the Lexar 2000x SDXC card...amongst others. 64GB can be had for 90 bucks, today, with a Lexar reader. Prices fluctuate...but, oh well.