In my post about Chen Bin's guide to using Gnus with Gmail, I mentioned that in my own quest to move my email operations to Emacs, I was looking at three packages: mew, mu4e, and gnus. In the comments, I got a couple more recommendations. David recommended Wanderlust as a mature and full featured solution. Sam recommended that I look at Notmuch. Both useful additions to my list and I'm glad to have them even though they complicate my decision making.
Sam also provided a link to a post by the invaluable Christopher Wellons that compares Notmuch and mu4e. Wellons' post is interesting because it's principally about moving off of Gmail and onto his own server that he would access using an Emacs-based email client. I found this particularly interesting because that's my end goal: no email middlemen that offer the NSA and others easy access to my email.
If you're OK with Gmail but would just like to compose messages in Emacs, Artur Malabarba has got you covered with his gmail-message-mode that lets you hot key from your browser to Emacs when you want to compose an email. Malabarba's got it working with Chrome, Firefox, and Conkeror. He uses Markdown to compose messages but it could probably be patched to use Org-mode fairly easily. In any event if you're interested in integrating Gmail and Emacs, give Malabarba's post a look.