I was an early adopter for the Mailbox app for iPhone (http://www.mailboxapp.com/). By early adopter I mean that I had my name in the queue before most folks so I was able to start using it almost right away when it was released in the app store.
There are a few great things that I loved about the Mailbox app, but that’s the problem. There were only a few great things…and that was it. I use Gmail for personal email, my small business and my day job (Google Apps for the latter two). I use labels extensively for grouping and keeping me on task. I have my day job setup in a manner that my email system works as a ticketing type system (I’m in IT) and Mailbox didn’t really meet my needs for that. The only real feature that can’t be had elsewhere is the ability to put an email on ice for a set period of time.
I had tried using the native Gmail app when it was first released in the iOS app store, but it was horrible. It was just a wrapper for their web interface (and to some degree it still is), had support for only one Google account, etc. I tried it again and I’m loving the experience! I get the same level of usability on my iPhone AND iPad that I get on my desktop. That’s something that no third-party email client will be able to beat IMO.
Mailbox is iPhone only and I spend quite a bit of time on my iPad for this sort of thing. Having a seamless experience across all platforms is spectacular. The wow factor of Mailbox wore off much quicker than I imagined it would and that’s ok, because I found something way better! Who would have thought that Google would have the best email experience on a iOS device? Obviously that only applies if you use Gmail exclusively, but that’s ok with my setup.