I've been dogfooding the "Gmail will double-check that your recipients all have access to any files you’re sending" feature for a month now and it's FANTASTIC. If you use Google Docs a lot, this saves so much permission pingpong.
Honestly Gmail's new features never cease to amaze me. Their new compose box is a good example. Initially it was terrible for me as I frequently send from other email addresses but within a week they had removed a click or two and now it's perfect.
I'm very impressed with how fast that other-email-address UI issue was fixed.