mail is (imho) clunky and slow, it uses too much processor and the sliding drawer is dumb - especially to real-estate challenged displays. It has problems managing multiple POP accounts (personalities).
eudora manages years of mail quickly and efficiently. I have six pop accounts all handled by one simple keystroke. The filtering system eliminates most of the junk.
The interface is admittedly corny and windozish, dated as they say, but I have it turned off and it is a clean, sleek client that works perfectly for me.
The only thing I'd like them to do is make compatibility with the system Address Book an option. At present, the ab in eudora does the job for me.