I've been using Thunderbird for a long time now (Netscape 4.x -> Netscape 7.x, Eudora 4.x -> Mozilla whatever -> Thunderbird).
Note the absence of M$ Outlook all along - for almost 6+ years now. I'll not go into why I don't use it except to say that it doesn't follow RFCs (open, common standards) properly and is a ripe cadidate for all virus, worm attacks.
I've dabbled with unix clients like pine, elm etc for a brief period of time. Didn't have the courage, patience to use/migrate-to GNUs or any other Emacs based package.
Anyway, coming to the topic at hand, I'm running Thunderbird for about 15hrs non stop and its currently at 93844K (94M) memory usage and still growing. That, IMO sucks.
Eudora 4.x was a wonderful lightweight version which never appeared to take anything more that 15-18M of memory. However, it lacked the ability to forward emails as an attachment (preserving headers) and that IMO is an absolute necessity in official world.
Anyway, if you know of something that's as flexible as Thunderbird, has all of its features, is _FREE_, let me know.