Actually, the primary binding is 'C-/', with 'C-x u' and 'C-_' as alaises. The 'C-x u' alias exists because it's easier for new users to remember and the 'C-_' alias exists because that's what some terminals send when you press 'C-/'.
I did end up with undo still on C-z after turning off cua-mode, because I use graphical frames and never want to suspend anyway. But it's not really a help: the C-/ C-? pair is much more convenient, and for nontrivial undoing I'm navigating a state tree anyway.