Airmail 2 is out now and it took me all of thirty seconds to decide this worth paying the upgrade pricing (currently sitting at £6.99/$9.99, 50% off)
With the interface enhanced for Yosemite and a wealth of new features (for the full list, click here) it's money very well spent.
I have been using Yosemite in it's beta form for a while now and I swore to myself that I would give Mail.app a decent crack of the whip. I wanted to see how much the Handoff extension would be of benefit to me and, in truth, it isn't. If I start an email, I'm generally going to finish it because I try to never leave a task unfinished.
However the bland interface just grated on me for far too long and the interaction out of the box that Airmail offers with my old favourites like Evernote, OmniFocus and Fantastical mean I've moved back to Airmail full time.
Check it out. It's better than ever