If Gmail's UI floats your boat but you prefer using a desktop email client for 'no waiting for a refresh' feel, Ken Mixter has developed a Thunderbird extension just for you. The GmailUI extension brings many of the shortcuts and search expressions of Gmail to Mozilla's Thunderbird client. Keyboard shortcuts like J and K for moving up and down through messages, as well as search expressions such as 'subject:' and 'from:' can now be a part of your Thunderbird experience. Ken even went so far as to include Gmail's 'archive' functionality and (oddly) a quick calculation function, right from within Thunderbird's search box.
The extension appears to be completely free, and Ken is accepting feedback and suggestions at his site. Head over to Mozilla's addons site to grab the extension, or check out Ken's site for a lot more details of everything GmailUI is capable of.