i've been using tb for some months now. never any update issues. the reason for my migration from mail was: mail.app couldn't handle my imap account and filled it up with 1000s of recursive copies of some stray message until it exploded... tb can handle imap nicely. its a clever programme with some very silly bugs: i can't define any helper apps (eg. acrobat or other viewers). the pref for this sort of thing has preview.app as helper for tiffs, thats it. you can change that or delete it - there is *NO ADD* button! doubleklicking pdfs will for some reason always fire up classic to start acrobat reader 5, no way to change that.
one other thing - but that may well be apple's fault - is, though tb is my mail client in the system prefs, hitting a mailto link in any browser will always open mail.app.