GPS is very reliable and very accurate; just make sure you have an external aerial (or at least a suction-cup one attached to the inside top of the windscreen). Get an aircraft-powered (cigar lighter) one with internal batteries and carry spare batteries. Get one with a big moving map; anything less is a waste of time.
Regarding "primary nav", it is a bit of a misnomer because one should always use two methods concurrently; in VMC it could be GPS+map reading and in IMC GPS/VOR/DME. In practice the GPS will be entirely reliable and by far the most accurate 99.9% of the time but you need to be able to get about adequately using the other method if you have to.