I switched from Mozilla Mail (1.7.3) to Thunderbird 1.0 over lunch; I can't say I've plumbed it's depths but I wanted to offer you some first impressions.

Praises:

• flawless install and upgrade experience; perfect import of configuration settings from Mozilla (Outlook Express also supported)
• very pretty default look-and-feel (nice skin with toolbar buttons, icons); consistent to Firefox
• Mozilla mail was already fast, but Thunderbird feels subjectively even faster: message display seems about 30% faster
• the search toolbar allows searches to be on subject, sender or message body; it also uses the edit area to show the current search type
• searches can be saved as folders; a saved search can span one or more folders and include multiple criteria
• much clearer layout for several dialogs, such as Junk Mail Controls
• the preferences menu item follows Firefox's example by moving to Tools/Options, in line with applications like Internet Explorer and StarOffice/OpenOffice.org

Nits:

• The toolbar button colours seem slightly out-of-sync with Firefox's palette (especially the reds and blues)
• the search folder uses a close toolbar button to return the view to un-filtered mode; I think a checkbox would be clearer
• I'd like to see search folders appear under a standard top-level Searches folder for ease of access and to clarify their operation (a delete on a search folder impacts the message in the actual containing folder)

Between Firefox and Thunderbird, the only thing that may keep some users from abandoning Mozilla is Composer; clearly the mail composer includes a HTML editor, but if you want to edit a longer document, you can use OpenOffice.org.

Firefox, Thunderbird and OpenOffice.org/StarOffice - now that's a killer suite.

Oh, and here are the other Thunderbirds.