Back in the days when I had a job that required using Windows at work I used Eudora exclusively so that I could easily move my mailbox files back and forth between Mac and PC. It's been years since I've personally used it, but recent encounters with Eudora on the Mac were a shock to the system. Always a powerful email client (behold the power of the X-Eudora-Setting), with a vibrant user support community, I was shocked that the interface had barely changed a bit in 10 years! Some might find that endearing but I found it jarring to have that particular UI - Aquafied just enough so that you didn't think you'd mistakenly launched Classic - floating in a sea of otherwise brushed metal goodness.
How many of you are currently using Eudora (paid or ad-supported) or plan to give it another shot when it goes open source?