Personally, the "Undo Send" feature is one of my all-time favorites in Gmail. A lot of people are comparing it to an outbox, but while the two are functionally similar, they are psychologically different. The undo send simply gives you a few extra seconds to re-read your email and catch any small errors before sending.

When the feature first launched, I was worried I would eventually adjust to it, and stop re-reading emails before clicking "send". So far, that has not happened - it's effectively a psychological trick that continues to make my emails better proof-read.