I use mutt because it's very very fast, and quite customizable (I maintain a kind of 0-inbox through a set of hotkeys, + spamassin + procmail). To handle numerous mailing-lists with medium volume, it's so far the best I've seen.
I tried many email clients (on Windows, OSX and Linux), but I always come back to mutt, because it's the fastest to handle the high-volume of email I have...