I've been using @delta for some months and I'm totally in love with it.
The concept of using emails as chats is equally simple and powerful.
And I love the fact that it just works as we are used to in other #IM apps, but without being locked to a platform, and having true encryption.
The onboarding process is extremely easy to do, and does not require personal data at all.
IMHO it is equally useful for non-experts and people who really need a secure and trusftul IM service.