In both campaigns I'm in (one I DM, one I have a PC), we have used some house rules and have used the "campain rules" in character builder. That said, the "campaign rules" tab really only good (from my fillding with it) at restricting where player's get options from - neither campaign allows stuff from Dragon, and using the tab means players don't have to wade through tons of options they can't choose.
What I couldn't figure out to do with it is to encorporate bonuses or other such mechanical benefits into a CB character. For example, I give all the PCs in my campaign +1 to hit/NADs at levels 5, 15 and 25 (while barring the expertise and paragon defence feats). As far as I could figure out, there was no way to do that through the campaigne rules - rather, the players had to go into the feats list, and use the houserule button to give themselves a bonus feat (either that or modify the character sheet itself every level).
So, I must admit, I haven't found the tab terribly useful (again, except for eliminating options based on source), but chances are I'm not getting the best use out of it due to not really being that computer savvy.
