What 'e said. DON'T DO IT!!!
Four years ago, I went from EudoraPro (paid) due to bloat and moved to TB.
This weekend, I decided to move on and after reading reviews of PocoMail and The Bat!, I tried out both of them and decided to make the plunge to The Bat!
The hardest part was figuring out how to export my TB emails intact. After I figured out that out (The Bat! docs on this are rubbish btw), I painstakingly exported all my emails (some going back to 1992).
The only thing I lost were my message labels but I can live with that I suppose.
As a commercial developer myself, Open Source is both a blessing and a curse. TB has become bloated and instead of focusing on fixing bugs and general functionality, they're just adding more bloat to it. I mean, who needs a newsreader in an email program? Some of the logged bugs go back to 2002 and remain unfixed.
People are constantly losing emails (I was one of those. Good thing that I'm a backup fanatic!), functionality etc. Its a mess. Trust me, you do not want to go there.
I just moved over 25GB of email. Not a hitch. 'nuff said.