Reading http://www.twistermc.com/blog/2008/02/01/outlook-thunderbird and you see limitations but the files do appear to make it over. Some complain about the "you may start to loose your folder structure."

Is it compatible? No. Can we import and export to and from OE and Outlook? Yes.

You don't need to move to Thunderbird. You can install it, import some stuff and see if it's better or worse for you.
Bob