A couple of weeks ago I gained access to the Mailbox app and I can't stop telling people how great it is. I've been able to stay on top of my mail and with it's reminder capabilities, I no longer have to worry about forgetting to respond to an email when I have time to give a thoughtful response. If you into GTD but haven't yet been able to conquer your inbox, I would highly recommend getting on the wait list for access.
Currently it works only with Gmail, but I've heard rumblings that it might add Exchange support in a future update, maybe as a premium service. I gotta say that I'll be quick to jump on that if I have the opportunity. This app actually makes you want to have email to deal with because it's so easy to handle with the reminders, lists, and touch based gestures.
Get it! Do it now!