First look: Thunderbird 1.0 a winner - Computerworld: "If you find yourself spending more time sorting your e-mail than reading it, consider the Mozilla Foundation's new Thunderbird e-mail client. The free program, which complements the group's Firefox browser, combines advanced e-mail sorting functions, first-rate spam filters and lightning-fast performance to help you cut through your in-box like a hot knife through butter.
I tested Version 1.0 of Thunderbird (a 5.8MB download). After years of using Microsoft Outlook, the switch was like trading in a big, lumbering Buick for a Mustang GT. A mere second after you launch the program, it's ready to retrieve your mail or compose a message. With Outlook, you launch the app and then go get a cup of coffee while it loads its numerous modules and applets."
No calendar (but the article also notes "although Mozilla offers a free calendar extension") but strong RSS channel support -- likely to grab major market share from Outlook Express.