Back in August, I came up with 18 reasons why Thunderbird was better than Outlook Express, which are, in summary:
- Themes
- Extensions
- Junk Mail filtering
- Better message filtering
- Not full of security bugs
- HTML Sanitization
- Sanitization for Junk Mail
- Cross platform
- Text Zooming
- Automatic folder compression
- Javascript Console
- Three-pane vertical layout
- Customisable start page
- On-screen alerts
- Message labelling
- Graphical emoticons
- Spellchecker
- Doesn't get hijacked
Now, in that article I was referring to Thunderbird 0.1, and since 0.6 is on the horizon I've come up with 5 more reasons that you should switch.
- Message Truncation - fed up of downloading 150KB virus-infected emails? Enable message truncation, which allows you to download only the first, say, 50KB of a message. If you want the whole thing, fine, click a link. Otherwise you can delete the message without it hogging your connection.
- IMAP IDLE support - a new feature for 0.6, this allows for better performance from IMAP servers that support it.
- Threaded message checking - I only noticed this after I had to use OE for something today. OE checks accounts one after the other - if the first mail server is slow, then you have to wait for it before checking other accounts. Thunderbird checks them concurrently, so one slow server doesn't affect your other accounts.
- Edit as new - allows you to edit existing emails as if they were new ones. I can't believe OE doesn't have this feature as I've used it so often. Nor can I believe I didn't put it in my original list.
- Ability to associate multiple addresses with an account - this is still a backend feature with no UI as yet, but it's possible to have multiple email addresses associate with one account. I have my Scrapie email forwarded to my Bradford email account, yet I can reply as if the account was separate. Let's hope that a UI appears for this soon.
I could also throw in that it's open source but I don't really think that is all that advantagous to the end-user. Still, I've never looked back on my decision to dump OE - possibly the only feature missing from Thunderbird is Hotmail support and there are already several third-party programs that will get around that.

Wilst I fully agree with Thunderbird being better than OE, as a tech support for a Microsoft loving firm I feel I should respond to your five latest reasons.
1 Message truncation is usefull in it’s place. It’s place is on pdas, webmail, and dial up. So yes it’s a good feature, but becoming less so for a mail client like thunderbird
2 Don’t know.
3 In an ideal world everybody should only need one mail server. And isn’t concurrent downloading just going to slow you down getting your important account on dial up? But yes, I do love this feature - my slow Russian server doesn’t slow down my fast uk server at all even when it goes offline for hours at a time.
4 Outlook has a resend command, admittedly not obvious though. I don’t know about OE.
5 Sounds great, but if like me you use authenticated smtp servers then the server balks at being given a from address on the wrong domain. Besides, if you have permissions then you can change the from address to anybody in the organisation in Outlook.
But I still love Thunderbird despite the flaws, there’s no way I’d ever go back to Outlook Express at home.
At work is a different matter. Webmail is best when I’m moving about, but I use Outlook 2003 when I’m at my desk.
I’ve been using Thunderbird to check some secondary e-mails for a while. It’s the second best e-mail cliente I’ve ever seen, and perfect for most people. But I keep sticking to my old Eudora.
Looks like you got the attention of Windows Zealot Paul Thurrott, who challenges you to do a comparison against Outlook (not Express).
Given that Express is wildly popular and Outlook isn’t (for good reason, IMHO), I think Thurrott’s just being his usual a**hole self. But how about it? Just for chuckles, take a shot at it.