I have used Mailplane on and off for years. I love that it turns the great Gmail interface into a proper desktop app, complete with drag-and-drop attachments, notifications and an icon in the dock and tab switcher. But I never liked its super power of spinning the CPU of my Mac at all times, even when supposedly idle.
Now v3.0 is out, and it seems to have solved the latter problem, while adding a few new features.