Being that mailbox is free it's at least worth a try. I tried it and didn't see how I could effectively incorporate it into the way I use email and ended up deleting it. I still use the Gmail app as my primary email client.
I also use the Apple Mail.app for a different account that I want separate from Gmail. I'd like to try Sparrow, but since it costs money and the Mail.app serves my needs, I've elected to stay status quo.
But I think you should at least try mailbox and see if you like it.