Inspired by f0dder's comments, I installed Thunderbird again to give it another spin. Moving mail from The Bat! is of course a pain, but it's manageable. What really gets me though is the stuff that should just work but doesn't.
For instance, I entered name and e-mail for an account, and the fancy auto-configure checked and created an IMAP account for me. Now the account was actually a POP account, but there was no way to change it back, and there was no way to do add an account manually without running the auto-configure. A little searching around with google (since the help site is quite useless) showed that you have to "press the stop button"
to stop the auto-config when it starts and then turn it into a POP account.
After adding a couple more accounts I wanted to move the last one up on the list to a different position. Drag and drop doesn't work, nothing in the right-click menu .. back to google. If you want to reorder the list
you have to install an addon or manually edit an ini file somewhere in your profile folder.
Then I exported mails into mbox format from The Bat! Now like f0dder says this is a very standard format, and is the format Thunderbird uses to store e-mail as well, so clearly it should be easy to import then? Alas, File->Open only reads eml files and the import wizard will only auto-import from other clients with no option to select and import from a file. Again you have to google and figure out that you have to install another addon
to be able to import from the programs own native format.
Thunderbird seems like a nice e-mail client, there is a lot to like about the filtering and stuff. It would be nice to move away from The Bat! (v18.104.22.168), which is an awesome e-mail client, but the yearly upgrade doesn't seem to really get you more than UI tweaks these days.
I just think they need to work on making the user experience feel a little less like a beta, and while relying on plugins to provide functionality is fine in some cases, there is a limit to what simple stuff that should just work you want to rely on some 3rd party developer to maintain and get to work when the next update is released.