Thunderbird is pretty good and I don't think you'll regret sticking with it, but after switching between Thunderbird, Entourage 2008, and built-in Mail, I think it's the lesser of the three.

Mail.app's comprehensive inbox view across multiple servers, integrated Spotlight, and "mac-ness" make it my favorite. That being said, I use Entourage because I need calendar support for Exchange accounts and iCal is too clunky.