I was there the day they set off their Nuclear Bomb, 2006 I think. As claims to fame go it is quite a good one and I do win the "I've been to a more remote country than you have" discussions 😉
As for getting off the beaten track, forget it, it's not going to happen and you will be escorted everywhere. If this is something that you can't cope with on a holiday then DPRK isn't the place for you. If you did manage to get away and were found out, not only would you be in trouble but so would your guide and your guides family.
It you are prepared to put up with being shown around and take most of the things you are told with a pinch of salt then it really is fascinating. Mental but fasciniating. You will have to like train journeys though as I think the only way in and out is the train from Bejing. There is an airport but there was an issue with plane inspections when I was there but this may have been resolved.
I've got some photos online but I can't access them at work.