Gmail has just added full-on IMAP support, and it's awesome. And by awesome I mean totally sweet.
This has pushed me over the edge. I'm finally ready to use Gmail as my primary email provider. Now that I can access all my email (including my old archived mail) however I want to from whatever client I choose to use, whether that's Gmail itself, Thunderbird, my iPhone, or good old Pine, Gmail is perfect.
Google, you rock.