I just gave up on Eudora, but only because I could not get the AOL server (holding my old-time Verizon accounts) down reliably, although I might have missed some tricks.
And I had a 1 year old TheBat! v. 7, and went to it. Overall, I actually like it, I am asking some questions on the forums. Eudora still shines in some areas, despite being 5,000 years old, but there are definitely some spots where TheBat! shines brighter. The chief deficiencies are that the "look" is not as nice, there are some places where Eudora has drag-and-drop and TheBat! does not, and the Eudora structure is more integrated when you have multi-mailboxes that really act as one. However, overall, I have been reasonably happy with the move, after about 15+ years of Eudora.
An example of TheBat! superiority is the virtual mailboxes., also some helpful message columns not in Eudora, and a flexible user-defined tab view of the messages, and all sorts of stuff.
I still like the Eudora filters a bit over TheBat!, but TheBat! is definitely usable, unlike the last time I looked at Thunderbird.
TheBat! opens multiple Windows-windows, Eudora worked in one. That is more a matter of style. Theoretically you have more power with more windows, but Eudoras simplicity and cleanness is a plus as well.
The Eudora forums are still very good, TheBat! forum has been very responsive, mostly one person doing a lot of care.