Mac 101 just published a great Mailplane review:
If you’re like me, you love Gmail but feel its missing key functionality which puts a damper on your productivity in using Gmail in the browser. Coming from a corporate world, I love Outlook for windows just as much as I love Gmail. Yet I’ve never been satisfied with Mail.app, Entourage, or running a Virtual Machine or Emulator in order to run Outlook for Windows on a Mac. So you’re stuck using Gmail in a web browser, somewhere it lies lost in tab oblivion… getting no respect. I’m here to tell you Mailplane will change how you look at Gmail and massively increase your productivity and user experience within Gmail. So here it is, Gmail as we know and love it. Looks about the same on the surface, however what you see before you is Gmail turned into an Email Client. Don’t get discouraged, its better than you might think. Notice the Toolbar at the top, from here you can snap a screenshot and automatically insert it into an email, insert media, switch accounts, or bring up your Contacts in Address Book. This is just the surface, so lets dive into some of the goodies in Mailplane app.