Actually, it works as is. A couple of notes on my experience:
1. I only tried it on a fresh install, so I don't know how it would go on an existing installation.
2. Don't export the users, otherwise you won't be able to log back in to WordPress. It's easy to skip - just don't click that link in the export script.
3. I had thought at the start that I would install b2, then upgrade that to WP to be on the safe side, but it's not necessary. Just install WP, run the export script, then run the b2-2-wp.php script (in the wp-admin folder) to convert the database.