I think "not ready for primetime" might be true. I installed it for a friend, and he keeps having problems, like
1) installing mime handlers seems to be hard.
2) the process of importing an address book from netscape 4 was messy but not Thunderbird's fault. Now he claims thunderbird is shedding addressbook entries.
and i think i might have the answer to 3, but:
3) he wants the cursor at the top, not bottom of replies with quoting, so he can intersperse comments.

in all i'd almost tell him to switch back to ns4 for mail :-\