I’ve never found a perfect mail client, but I’ve used KMail off and on as my primary client since 2000, and it’s definitely my favorite.
I strongly prefer IMAP, and only two clients I’ve tried have handled IMAP at all decently: Mail.app on Mac OS X (just about the only Mac OS X application I might reasonably miss) and KMail. My most recent flirtation was with Evolution, but as my mail folders grew larger and more numerous, its performance went from adequate to abysmal and, finally, unusable.
When I switched back to KMail, I set up my IMAP connections as cached IMAP connections, and my problems went away.
I have a few small gripes and questions that I really should ask on a #kmail or #kontact IRC channel sometime, but it’s only when something goes seriously weird that I even have to think about my mail program. That’s the sign of a quality program.
Thank you to all KMail and Kontact contributors and developers!