For myself, I find that Thunderbird is pretty much the only decent game left in town (at least under Windows). I gave up Eudora for Thunderbird, and would never go back. I've also used Outlook and Outlook Express at various times in the past, and I won't go anywhere near either of them anymore either. I also tried Pegasus Mail earlier this year while searching for a Eudora replacement, and the interface was so screwy that I gave up on that one too.
I kind of liked KMail, but that's Linux-only as far as I know.