You can let those tears for Sparrow dry. Like Sparrow, and even more so, AirMail is an email app rethought for the Twitter age. Simple, elegant, powerful only when you need it to be. It makes it hard to complain about using email. And most importantly, it's maintained at an outstanding pace by its development team.
At the time of this writing there was no stable 1.0 release for AirMail. I'm confident however that it will be released soon and be commercially successful enough to sustain it. Or better yet, prevent it from being acqui-hired by the Gmail team like Sparrow was.
Important: Airmail doesn't allow you to send secure encrypted email messages yet, I would highly recommend checking out GPGTools if you use email for work and often need to communicate about sensitive information or share credentials (usernames, passwords, API keys, etc.).