I am thinking about learning Cocoa just so I can write a Mac e-mail client that doesn't suck.
Eudora sucks but it is the best of the bunch, but it is buggy on Intel for me. My Inbox keeps losing memory of labels and what's been read (even what I have already filtered out).
Everything else is pretty bad in various ways. Mail is not a serious mail app. Thunderbird isn't a Mac app at ll. Mailsmith doesn't do IMAP and is bloated. Google Mail isn't local.
Maybe I'll just give up e-mail altogether!