I guess I'll agree that most people don't understand GPS. What most people call GPS is a GPS receiver tied to a navigation package of some sort. GPS is nothing more than a positioning system that works (very rough description) by comparing a set of timing signals against a "catalog" of satellite data. The accuracy of the timing data (and the catalog) along with the distances involved allows the position and probable error values to be calculated. Height is always less accurate than lat / lon due to the geometries involved. All a GPS can tell you is where you are or maybe some track and velocity information if you integrate position data over time. Getting access to more accurate timing data can dramatically improve the position accuracy as can some other tricks (known as WAAS and LAAS).

Every GPS I have ever used (even the horrible navigation system in my wife's car) has provided (somewhere) an indication of how accurate its positioning is. Figure out where that is on your system. As long as you know the error margins and characteristics (i.e lateral is considerably more accurate than vertical) even commercial GPS is a decent and reliable system for figuring out where you are.

Most so-called GPS errors are actually navigation system or map generation errors. I might trust the routing of a commercial avionics Flight Management System which is designed with a lot of safety checks and subject to FAA certifications and audits. I would be less trusting of a military navigation system and trusting a navigation package in an car or hand held navigator that does not require or provide documentation of similar pedigrees in development or testing processes is just not an option. Of course the car/handheld nav systems are fun to play with and can provide good situation awareness - just don't let them think for you.

-Eric