I am a ruthless inbox zero fanatic. While the built in Mail is good for this, Airmail takes it to the next level with "send and archive" support as well as syncing signatures across platforms. I can tweak my flow, keep everything in order no matter what device Im on and focus on my email.
The app does have some rough edges but its getting better just like the OS X version did. Sherlyn helped me out with an issue I was having in a few short messages which I find really impressive.
JiveMasterT about Airmail - Your Mail With You