Okay, I've been running Jamaica Plain on this build of Windows 10 for a couple of days now. I wish I had an automated suite of tests to run to validate the behavior of Sonar on these Insider Preview builds, but I don't so I'll leave that to the folks at Cakewalk. In any case, I've recorded vocals and guitars and used MIDI key boards through my MOTU 8pre with Windows 8 drivers. I route guitars through Guitar Rig 5 and I use MIDI to drive AD2, Rapture and Dimension Pro successfully with no issues. I've edited in both track and PRV with no hiccups of any kind. I've rendered projects in a variety of formats as well.
I have not used any of the new features such as patch points yet, I'm still considering how to use them because I know I'll make a big mess if I don't do some planning first. Anyway, within the scope of what I use, it all feels very stable and quite solid to me. The startup performance of Sonar is much better, which I attribute to the new JP release improvements there.
Bottom line, Windows 10 10565 and Sonar are a fantastic fit.