I recently purchased the MX Master 4, and it was easy to remap the gesture buttons and configure features like SmartShift and high-resolution scrolling.
One deal that I haven't dived into -- what is up with the ones that appear to limit how many times you can change devices? Is that real and not hackable?
(Unifying receiver does not unify unifying and bolt so not so unifying eh! Oh and bolt is newer but not backwards compatible, so annoying!))