I've long found Gmail's Undo Send feature to be very useful. I often send without fully proofreading, and this gives me a chance to retract and correct a message before it really goes out. Nice.
Send and Archive is also a nice feature. When replying to an e-mail, rather than separately sending and archiving, one can send and archive with a single click.
The rub, however, is that send and archive doesn't have an undo control, and so now if I fail to proof a message, tough, it is gone. Ultimately, one has to decide which feature is more important, and I'd rather appear literate than save myself a click now and then (well, several times per day).