Enabling Hot Reloading (w/ Ctrl+D) lets you see changes as you type, you don't even need to hit Save. If this is a sign to come from React Native, it's going to be a game changer for both iterative development experience and code-reuse between iOS/Android.
My only concern is that I don't see how an IDE not developed in partnership with Facebook is going to succeed as I'd expect it to always be playing catchup with new features. Maybe it would've been better as a series of Atom plugin enhancements on top of Facebook's https://nuclide.io