The lack of imap IDLE is a killer for me. I don't want to poll the server all the time, but I want to be notified of email ASAP.
Offlineimap also breaks with imap IDLE, regrettably.
That's a good question. It's one of those things that I remember being complicated, but when I look over my configuration, it seems pretty damn straightforward. I guess the fact that Channels and Groups share a namespace irritated me?
Anyway. It's a great piece of invisible, blue-collar software.
It's a tiny C utility written by the mutt creator, Theodore Tso and others, so it's very good as expected.