(As a mea culpa for the spelling errors in the subject line of a couple of recent posts, I’m going to skip politics today and try to be helpful.)
1) I’ve been using Protonmail lightly because it’s kind of a pain to use a temporary e-mail address regularly. One of the coming features is support for user domains and, as soon as that is available, I plan on making Protonmail the home of my primary email address. As it is, I’m only using email@example.com sporadically.
2) The interface is quite nice and intuitive. I wouldn’t say that it’s markedly better than other webmail products, but it’s more than good enough. I do like it better than GMail’s interface and I used Gmail for years.
3) Given the light usage, I haven’t come close to bumping up against the message or storage limits. Looking at my other accounts, I doubt I will any time soon even when I move mail.wtfpancakes.eu over to it.
4) It really needs a critical mass of users. I haven’t come across very many other users of Protonmail so many of the features are currently collecting dust. I can confirm that the “encrypt for outside users” function works reasonably well, although I can imagine the recipients finding it inconvenient.
Honestly, thus far, I’ve only taken it for the equivalent of a test drive; I’ve yet to really take it out on the highway and see how it performs. So far, so good, but “so far” isn’t terribly far yet.