Not sure if you guys know, but Eddie Cue's son works at Orchestra. It wouldn't surprise me if Apple ends up buying this company. I think they should. Their Mail.app client feels so far behind compared to Mailbox.
From what I can see of Mailbox, it looks fantastic- but it's not for all users. It's adapting e-mail to a specific usage pattern that many of us will have, but few non-professionals would use. So I don't think it would replace Mail.app.