Thunderbird with imap is good software. For about 13 years I've been using the same email client: vm in emacs. While reasonably quick and infinintely adaptable, it's really not suited to modern email with attachments, HTML formatting, and multiple servers.
I just switched to Thunderbird and Dovecot imap and it's like a whole new world, simpler and neater. I now have much less reason to fire up a vt100. All I do in emacs now is coding and blog posting. I'm still using procmail and spamassassin for mail processing, but wondering if I can dispense with some of that, too.
Imap is really great, the server-side model is definitely good. But it doesn't go quite far enough: imap should provide mail delivery and address book management too. I don't want to have to configure ldap and smtp also.
What I'm really missing is the ability to easily search my 300 megs of email archives going back twelve years. Despite having written two mail search systems, I still go back to grepmail on the raw files. Gmail is cool and all but I don't like browser hosted email.