New Facebook Messaging is awful. Can't branch/split conversations. Can't hide or even remove parts of conversations. Newest responses appear at the bottom which is completely the opposite of how new messages often appear in email or in the Facebook news feed itself. People who email you from outside don't cause the "new mail flag" to flip on. (Maybe it should be blue to indicate external and purple to indicate mixed?)
You do get your @facebook.com email address, but that's about it.
Not exactly sure how this is an improvement. But if you're not in yet and not on the waiting list yet, I do have two invites to share.