On the subject of GPS, as a controller located on the eastern side of this debate, I have watched many aircraft on radar trying to make their own way to the ILS or something similar. Some have done things that you wouldn't believe - and that I certainly didn't expect from an aircraft that wanted to route the most efficient way to a point! Whilst I've no idea what the reason for such deviations was, I've seen it often enough to not blindly trust RNAV systems. True, pilots doing 'conventional' navigation have also done similar things but not nearly so often.