According to Adrei Lankov, who wrote This book on North Korea , it is estimated that 1 in every 50 DPRK citizens is a member of or reports directly to the Secret police. It is NOT that there is no resistance, but the rewards for turning in those organizing or involved in anything against the state is lofty. Like, your children could go to a better school, or you receive better state perks ect ect. So anyone who is even suspected of being anything but eternally devoted to Deal Leader will have a price on their head retaliative to their indiscretion, and just about anyone would sell you out if it means making life less miserable for them and their family.
Trouble is, the people that are most capable of revolting against the regime are often members of that regime. I read a book that speculated that even if these individuals were able to plan a coup to overthrow the Kim dynasty, they wouldn't since they could be implicated for human rights abuses ans subject to consequences.
The regime is not sustainable in the end, but its collapse has been predicted since its inception, so its just a waiting game.
http://www.amazon.com/The-Real-North-Korea-Stalinist/dp/0199964297