Mailbox is a new email client for iOS and Gmail that is currently running a staged alpha program. Upon opening the app you get a welcome screen showing you your place in the queue and a realtime ticker of the people behind you. In short it's growing like a weed. Between last night and today, the backlog has grown to 212,000 people behind me.
People are clearly not happy with the native mail client on iOS as demonstrated by the growth of Gmail's app but Measure proves that isn't good enough either. I myself find the Gmail app a huge improvement on native mail is iOS6 but still far behind Android. As discussed on This Week In Startups recently, we may well see Apple acquiring some of the rising stars of the ecosystem to prevent Google from eating the iOS ecosystem from the inside out - you only have to look at the top 10 apps in the App Store to see how that is a real threat.