Exciting news! Roundcube web-based email is now available in the debian repositories. I first used roundcube thanks to the fine folks at thirteen, and have previously tried it on a manual install. Both times it worked fine, but I continue to use squirrelmail for its reliability.
Now that roundcube is in debian, I'm leaning towards using that instead. I also noticed it can use sqlite as the backend, which makes it a super easy setup. I'll probably opt for mysql so that the web-based side of the email service can be made more distributed.