As far as I have tested it it makes you only have to login at the Drupalwebpage, as long as your users at the Drupal installation and punBB installation are equal (Username and password), or you are making a new user at the Drupal webpage.
If you are running Drupal 6 you will have some problems with the settingspage if you don't upgrade your forums to punBB 1.3 and installs an updated version (See a link he posts later in his thread and use Google Translate to translate it to English.)
You will also have some problems if you use a prefix and/or are not running the Drupal installation in your webroot and your forum in webroot/somefolder. This is fixed in his updatet 1.3 version. There is also a minor problem with some HTML, but it is just to add <ul> and </ul> in the right place in the function that makes the newest thread block.
It does not import anything, so you still have your punBB forum and Drupal installation as defualt. (See my page, beta.aaslan.net, it is in Norwegian but anyway.).
I have tested it with the newest version of fluxBB 1.2 I think, and fluxBB 1.4 with some modifications and it works well. Maybe I will release my update if he says it is Ok, but i gues you can fix it on your own if you have a basic knowledge of HTML