1) How close in functionality is the Forum plugin to bbPress?
2) Why would I want to use it instead of bbPress?
3) Have you folks gotten together with the folks at http://www.tapatalk.com to see about getting the Forum plugin added to the list of forums they support?
If not, then you really need to look into it. Forums are fairly difficult to use on a mobile device and being able to add the TapATalk plugin to your site so that it works with their mobile app is a real improvement for the user experience.
They already support bbPress so it might not be hard to get them to add support for the Forum plugin (or if it is close enough to bbPress there might be no work at all – just getting them to add the WPMU Forum plugin to their list of supported forums.