Glen -I signed one when I had a payout from another federal government agency. The non-disclosure was very strict and I tried to abide by it.
This did not stop employees of the agency trying to stop my appointment when I found another job, fortunately my new manager trusted them as much as I did so their "advice" was rejected.
Reason - they have the money to take you to court if you disclose - they also have the money to fight you to death in court if you try to sue them for disclosure.