I've used it, but only for testing. It works as intended. That means that it sort of kills the FB session data in Android. I'm not sure if it does the same in iOS. I believe the functionality there is contained in a precompiled file.
What are you expecting it to do?
Once you login with FB with an app it should create a basic permission dialogue. After that permission is granted the user will not see that dialogue again when FB login is called. If you're testing and you want to see the dialogue again then you need to go to Facebook and go to your settings then apps and revoke/delete permission.
Also, if you're using FacebookPostToWall on Android you might want to check out my fix
naplandgames.com/agk
Right now on AppGameKit Android apps, Facebook will ask you to enter a username and password when you attempt to post to wall no matter if you are logged in or not. This is because AppGameKit does not ask for permission to post to wall. This permission needs to be asked for separately from the login. The guide on my site explains how to modify AGKHelper.java to do this. It's about 5 lines of cut and paste code. Very easy fix for an ugly problem. Paul is going to wrap it into the next AGK.
Let us know how you make out and let us know the specifics (what platform you're having issues on, what you expect to happen) and I'll see what I can do to help!