I stand by my words, never regretted clicking send. This is a feature for people for whom 30 seconds is long enough to change their mind on if they have something to say. Maybe they could think for 30 seconds and ask two questions.
Well, a few years ago, I'd have said the same. But then I got involved with several of the latest "smart" phones and tablets. As a result, I now think "Undo Send" sounds like a fine idea.
The reason, of course, is all the times I've been typing a message, when suddenly it blinks out in mid-word, and I find that the partial message has apparently been sent. My muttered "WTF!?" has no effect. I've generally had no idea what I may have done (if anything) that caused the software to act that way. This happened once today on my Android (HTC ONE) phone, and I've seen it on several iPads and Android tablets. My wife reports the same behavior on her iPhone.
Of course, this wasn't a case of me clicking Send, so perhaps your "never regretted clicking send" does apply. But it'll be useful if a Send triggered by the software itself when I didn't want it to send anything will suffice as grounds for wanting an Unsend capability.
The only problem is the 30-second window. The email (and IM) interfaces are getting progressively more baroque, and that may often not be enough time to understand what has gone wrong inside the goofy software. What we really need is a way to tell it "Don't ever send anything unless I explicitly hit the Send button." But the clever software "designers" also seem to be eliminating things as mundane as buttons with words on them, replacing them with idiosyncratic icons (different in every email/message app) whose behaviour can be hard to remember if you routinely work on several different machines, as many of us do.
(Just today, I tried to back out of a messaging app by using what looked a lot like the usual left-pointing "Return to previous screen" button. It sent the message, though I'm not sure who it went to, and I hadn't even intentionally been trying to make a reply. Things really are getting this messed up. ;-)