The "Penelope" project's intention is to join the EudoraŽ user experience with the Mozilla platform. We intend to produce a version of Eudora that is open source and based on mozilla and Thunderbird. It's *not* our intention to compete with Thunderbird; rather, we want to complement it.
We are committed to both preserving the Eudora user experience and to maintaining maximum compatibility, for both developers and users, with Thunderbird. It is our goal to build a single development community around Thunderbird and Eudora, so that both mailers advance faster than they previously have.
The fact that the project is hosted by mozilla.org is the biggest clue - this is not an open source release of the existing Eudora program as such, but a new program based heavily on the Thunderbird base.
So the question is, apart from some import tools, why not just switch to Thunderbird?
I use Kmail (part of Kontact) in Linux (KDE desktop). I prefer it over both Thunderbird and Evolution (both for the Gnome desktop).