Back in the day I used Eudora Lite on my MacOS 7 machine (a 16 mhz LCII with 4 megs!) while SLIP'ing my way onto the Internet. I stuck with Eudora for years, until the Mac version of Outlook was robust enough to pass as a real Macintosh application. I like the three pane view over Eudora's method of individual windows for mail boxes (the usefulness of the single window interface is proven imho). Then Entourage came around and had a calendar, notes, kickass filtering, and so I went for it.
Mail.app is for two types of users, those who buy Macintoshes because they are easier to use and that person needs that kind of help, and power users who are procmail'ing their mail and just using Mail.app as the interface (which is a pretty cool way to go about it actually). But I don't know procmail (I'm a UI guy, and I've picked my battles) and need something more robust than Mail.app, so hello again Entourage, I hardly knew ye.