I train and assess all levels of NNAS and would say they are a superb way to learn. For too long people have had compasses and numbered bearings and the like thrown at them straight away, when actually it is far better to start by really getting to know the map. Of course, by you get to Gold you are navigating between little contour wiggles at night on bearings and pacings.
They focus on nav, but also cover safety, kit and environment.
There is a close link with the Mountain Training leadership courses, too. Bronze NNAS is at Technical Difficulty 2 and is the standard required of Lowland Leaders. Silver is at TD4 and is the standard required by Hill and Moorland Leaders. Gold is TD5 and is the standard required by Mountain Leaders.
NNAS is recognised by Mountain Training, British Orienteering and the Royal Institute of Navigation, along with many others.
There are also the Mountain Training Hill Skills and Mountain Skills courses. These cover a lot of the same ground but are not assessed, and thus aren't qualifications in the same way as NNAS awards are. Speaking to people having done different courses enjoyed the Skills courses but on the whole those who did NNAS felt more confident in their navigation.