I just did a test import from 6.5 to 7.5 and I see 2 issues so far.
I all the users of the board now are listed as Moderators or at least under Group Management there are now over 7200 Moderators.
I think had to go through every forum and fix the permission because a lot of them there was no values in the field. So I had to enter either a 1 or 0 into the field. Once I was done with that I tested a few of the members accounts and they could not see any of the forums. All I was getting the message "There are no forums for you to view." .
All message and forums imported over fine. I can log in as one of the Admin users and see everything. I double checked all the permisson give access to see the forums to all groups but that did not change a thing. I still can not view any forums.