I installed Thunderbird for a few mintues, mostly to test the RSS but without OPML import (as Moshe Eshel mentions here) I didn't bother. Even if I only had 10 feeds, entering them by hand would be too much trouble when I'm more or less happy with my current reader.
I decided to test the mail client (since that is the main reason the product exists) and went to the "import" option. I wanted to import my Outlook Address book (and possibly the settings) but regardless of whether I checked "addresses" or "mail", when I checked "Outlook", it always said "import mail and addresses from Outlook" which is not really a viable option. Importing the last 4 years of email from outlook seemed to work but was not at all what I wanted. I am impressed that it managed to pull in the dozens of mailboxes and the messages in them, so if you want to move away from Outlook from some odd reason, Thunderbird may be ready for you. I don't plan on leaving Outlook any time soon, though, and without OPML import, Thunderbird just doesn't have anything to offer me.
However, Thunderbird isn't made for me, it is made for Outlook Express users. I know plenty of them and will probably give Tbird another run later so I can see if it would fit their needs.