I've been using Airmail as my only email client on the Mac since I finally abandoned POP mail about three years ago. Simplifying how I managed email across devices meant funneling everything through a single Gmail account, and Airmail was originally designed to operate solely with Gmail. Since then, it's gained considerable new powers, including the ability to deal with other account types (although some of its Gmail-centric assumptions got me into trouble when I first tried to use it with an Exchange account).
Once again late to the party, I've just recently tried Airmail for iPhone. I was a little worried what form it would take, since I don't find the desktop version to be a particularly Mac-like app in terms of interface; it does offer tons of power-user customizability, though. There is some family resemblance, but it's clear that Airmail for iOS is a brand new app, in some ways outpacing its Mac predecessor. And since it no longer makes sense for Airmail to take all of its cues from a single email platform (Gmail), it's taken its cues from the app platform (iOS).
Airmail adds splashes of color and meaningful iconography to the bland table view offered by Apple's Mail app. But the true power lies in Airmail's ability to process email with swipe workflows and an in-message action menu, which can be tweaked to create a unique configuration for every user. The action menu slides in from the right with a single tap and bears an uncanny resemblance to the action menus in Drafts, another standout app for iOS power users.
Even Airmail's shortcomings have been blessings in disguise. On launch, you are taken to a unified inbox that combines messages from all of your accounts. You can exclude accounts from this view, but unlike on the Mac, it can't be disabled entirely. I excluded all my accounts, thinking it would be the same — it's not. While this means extra taps to see email, it also means that when I launch Airmail I typically don't see any email. Airmail's declaration of "Inbox Zero" is a white lie, but it would make the coiner of that phrase, Merlin Mann, very happy; he's gone to great lengths in the past to avoid the randomness of the inbox when trying to accomplish a targeted email task, like writing a new message or finding a particular piece of past correspondence.
I know I've only scratched the surface of what Airmail for iOS can do. It's annoyances won't come from the app, or the platform, but from the aging, overburdened medium of email itself. Against that powerful force, at least on iOS, Airmail is doing the best it can.