I've already replaced Mailbox with Boxer.
Boxer supports my self-hosted IMAP accounts.
Mailbox...still doesn't.
Boxer supports Exchange accounts for my corporate e-mail
Mailbox...still doesn't.
Boxer lets me customize the actions taken by each swipe gesture.
Mailbox...still doesn't.
Boxer supports Box.net as well as Dropbox.
Mailbox...never will, since it's owned by Dropbox.
Mailbox did one, and only one, thing better than Boxer for me: it provided a feature to automatically archive all the mail currently in my Inbox when I was starting up, thereby making it super-easy to start at Inbox Zero. That was a nice touch, but it's a one-time feature.
For the things I do everyday, Boxer has everything I need.