I find that it hasn't really been designed for keyboard-only use. Some features, such as switching folders for instance, seem to require a mouse, or at least I haven't been able to find a shortcut for doing it. It's also kind of hard to avoid accidentally composing html emails - it cannot be disabled completely. I still use thunderbird myself though, as I haven't been able to find a better alternative for Linux. claws-mail comes close, but it does too much work in the ui thread, which grinds everything to a halt whenever it fetches email. Why do mail clients all seem to suck so much, given how much people use email?