Since 2005 when I switched to GMail, I was living in browser. For better or worse, I learn to put up with issues like speed, lack of hotkeys, UI that is not as responsive as it should be, drag and drop that works most of the time. To be fair, in these five years browsers and GMail UI improved tremendously. It was rational decision to trade all these for simplicity of the UI.
Even after switching to Mac in 2006 - first for home, a year later for everything did not change this, I kept using browser despite better choice. Apple Mail may be decent client but it never felt quite right and otherwise decent Thunderbird had always issues with trading off multi-platform reach for integration and true Mac-ness.
All of this changed today after installing Sparrow. It is all I ever wanted - IMAP based access that really works, with minimalistic but elegant interface, Twitter like simplicity and that special sauce which discriminates great OS-X application from average. For very reasonable price $9.99 available in App Store.
99% of all GMail features are there - the only one I had to sacrifice is multiple signatures for single account. Before, all my work email was redirected and merged into my private GMail account, set up with multiple aliases and GMail was smart enough to pick different signature based on sending address. To do the same, I had to cut the redirect. Luckily, both Sparrow as well as iPad mail client support multiple accounts so I have access to all accounts inside single client on Macbook or iPad.
Sparrow - http://www.sparrowmailapp.com/, available in App Store.