I've been a diehard Eudora user since the early 90's. But have switched to Thunderbird in the last couple of months. Although it hasn't passed the test of time with me yet, so far it rocks.
It has successfully gotten a handle on my spam and handles my volume and all of the different accounts I have wired up flawlessly. It even imported my years of Eudora mail with no problem (though I did have to let it run overnight to do that as I also have mail in those boxes dating back to the early 90's).
No real faults with it so far, learning curve wasn't that tough as it is kind of a blend of Eudora and Outlook in terms of look and feel.
It's also from the Mozilla foundation, same people who do Firefox:
http://www.mozilla.org/products/thunderbird/
Same client is also built into the latest Netscape suites, but I prefer my email to be stand alone.
My second choice for email client these days would be web based....GMail.