I've been a devotee of the Sparrow mail app for Mac since it first came out. Its clean interface and seamless integration with GMail put it head-and-shoulders above Apple's own Mail application.
Unfortunately, those days are numbered - Google's recent acquisition of Sparrow is a near-guarantee that the application will receive no further work, beyond promised bug fixes. The timing couldn't be worse - with Apple having just released OS X Mountain Lion, there are bound to be problems with third-party applications, and it seems that Sparrow is already showing some.
I've been finding the UI locks up on me when composing or sending emails; labelling emails I want to keep isn't happening, nor is the deleting of emails that I don't. I can receive emails OK, so it's not a problem with my network or account. Quitting the application tends to cause it to hang, until I kill its process with extreme prejudice. And it's not just me who's seeing this:
All in all, the sort of experience that third-party developers would be rushing to fix; but with Google now calling the shots, how quickly will this happen? I'm not holding my breath.
Meanwhile, in the interest of actually Getting Things Done, I'm making do with Mail again.