Just got the June 3rd build of Thunderbird for Windows. I must say it is really looking good. It's fast (loads in a split second - much faster that Outlook Express), plus it does have a Firebird-feel to it that is strangely appealing. It also allows importing of address books and mail accounts from Outlook Express, Outlook and Eudora (only tested importing from Outlook Express, and it works perfectly).
Now that Firebird is my browser of choice, I'm really looking forward to an email client that has a similar appeal, and Thunderbird will be a big player in the email client scene in the near future as I foresee it.