I've been using Netscape's mailclient since ver. 1.1 (way back in the day), and haven't seen any reason to change... It has all the features I want and use, doesn't take too long to load (as I've allways got a copy of Netscape loaded), and doesn't have any trouble with handling a lot of email... (some of my folders contain more than 66k emails!)
Used Eudora on the uni for a while, but didn't like that too much either... Maybe because it made using pine in the same home area (though under different OS) a pain...
And on *nix machines, I use Pine... It does just what I want, and nothing more...