Well, Zendo is not open source, and it uses "one time pads" - classic signs of snake-oil cryptography (Vernam ciphers are information-theoretic secure if done properly - but impractical for almost every use case, because they really just amount to secret-splitting - and the most fragile crypto primitive, because if you do even one thing slightly wrong, they're awful). It also doesn't look to be forward-secure, unless I'm misunderstanding something from 2 minutes of research. And it needs manual setup.

So very poorly, I'm thinking.

TextSecure/Signal can present/scan a QR code in-person to do a public key fingerprint verification that way.

I'm sticking with TextSecure/Signal, thanks.