A couple months ago, I setup an install of WordPress + BuddyPress + the Boss. theme for community.codetn.org. Among the many BuddyPress features that we take advantage of over there is the messaging feature.
Out-of-the-box, BuddyPress + Boss. provides a very vanilla message composing interface. Now the makers of the Boss. theme have released a new plugin, BuddyBoss Inbox, that turns BuddyPress messaging into a Gmail-like experience.
Turn your BuddyPress messaging into a state of the art inbox. BuddyBoss Inbox enables attachments, labels, drafts, auto-save and a visual editor right in your BuddyPress Messages.