Newfydog is mixing up short term repeatability with accuracy. Even if his unit did repeat to 2cm in the time it took to finish his beers, he could still have been off in absolute position (accuracy) by up to 40m. I have seen errors that large on my Garmin 60cs (a good civilian GPS unit) especially in canyons or mountainous areas. On the other hand sometimes it is good to 3m if I have a good lock on 4 or more satellites in open terrain.
In addition to the GPS accuracy you have to RSS (square root of the sum of the errors) in the mapping error of whatever maps you are using as others have mentioned. There are other error sources such as poor satelite coverage or multipath errors that would come into play if you are in a canyon or region of other steep terrain. Even if you have occasional accuracies of 3m, on average the error is going to be much larger. Using a GPS to navigate while driving in a whiteout would be a really bad idea. It still however would be valuable for telling you what you are near like an exit or rest area.
By the way, the geolocation accuracy of the Worldview 1 satellite imagery used by Google maps (my company built Worldview 1) is about 5m for those of you who like to overlay satellite maps with GPS tracks. Not all the imagery used by Google is that good though.