A big minus is, the way Mailbox handles labels. It doesn't support the existing GMail labels. Mailbox creates its own lists in GMail. If you use GMail labels extensively like me, it may be a strong reason for you to stay away from this app for a while.
As a workaround, you can move your existing labels under Mailbox label as nested labels to use your existing labels in GMail. GMail will automatically update all the filters associated with that label. But, I will wait for Mailbox to fix this.