Canon UK recently lent me an R5 to try out with tilt/shift lenses. Only a relatively short look, so I've stuck to looking a it from the POV of a 5Ds user.
The TLDR is that it works a treat with them, image quality for my (base ISO) use is a tad better in giving a bit more room for adjustment.
I also tried it with the polarising filter adapter - works well, even giving me a usable (from my POV) polarising option for the TS-E17 albeit with all the concerns you'd always get with a polariser at such a wide angle.
What I really liked was the usability of the camera, the EVF and sensor IS - sure, a few gripes but nothing major (remember the limits of my interest ;-) )
Even my old M645 lenses get a new lease of life with a Fotodiox t/s adapter - with added IS too...
Anyway if anyone is curious, I've written up some details at
http://www.northlight-images.co.uk/looking-at-the-canon-r5-camera/...and I've even made a video about it
https://youtu.be/mx4_8qG-5hIBTW videos to supplement the articles are a very new thing for me. Someone at Canon finally persuaded me put aside my longstanding personal dislike of video as a medium