Mailbox and this solve completely different problems. I love Mailbox because it creates the notion that "read" does not mean done (and I can ask it to remind me about stuff). It also makes my email my todo app, which is nice since I only have to have one app. This is a really powerful change to the way i've used email and has made me more way more productive and organized.

SquareOne looks really interesting, and I think it and Mailbox could complement each other, but to me SquareOne is super smart filters for mail, while Mailbox changes the way I use email in its base form.