I've been using Eudora since it's very earliest days, and I still like it's approach to the task and the way it saved messages using flexible filtering to store messages anywhere you wanted in a directory tree-style mailbox structure. Web mail labels and such aren't the same and the clients that mimic web mail, which is how Thunderbird feels to me, simply don't o the same job. Plus, I'm lazy and don't want to change how I think about email.
So I'm still using Eudora, the OSE version now. However, it hasn't been updated since the end of 2010 and I have a feeling it may not be again - a dodo that forgot to fall over. OSE is only Thunderbird anyway, just Thunderbird macro'd to the gills to look and act like Eudora... if Eudora got drunk last night and so can't help being all sluggish and out of sorts today. Macros are great for some things but, as the main user interface, they can't help being what they are: a cumulative kluge to make one thing seem like another without any sort of actual core transformation. At this point I'm only using it out of habit.
Now I have to try to say "cumulative kluge" ten time fast.