I want to like Thunderbird, I really do (I love Firefox and have contributed code to their source trees). I've tried it many times, but there are just a lot of little quirks--not things that are necessarily good or bad--that keep preventing me from adopting it. With a browser, the browser's UI mostly stays out of the way, and you're dealing with just the content and the UI that the content presents. With an e-mail client, the UI is much more important, and having been an OE user since 1998, I just couldn't get into it.
So I'll also toss in a Thunderbird vote out of principle, but my preference still lies with WLM2009 because of a decade and a half of ingrained familiarity with the specifics of that UI.