This is a tricky one...

...IF you need IMAP. If you don't need IMAP, get Mailsmith. End of story.

If you do need IMAP (as I do), I wouldn't spend a penny on any of the current crop. Entourage is slow and comes from MS. Powermail is fast but has hardly any IMAP features. Mulberry does everything right but is a UI nightmare. Eudora is the worst soft of compromise. Overall, the best two IMAP clients on OSX are IMO, Mail and Mozilla. Mail wins on the UI hands down, but Mozilla supports more IMAP features. Both are free.

I expect the landscape to change when Bare Bones get IMAP support into Mailsmith.