As this seems to hve turned into a general discussion on GPS use....
One of the problems I have found is it is normally so reliable that people have difficulty in beliving it can, and does, produce inaccurate positions from time to time.
A handheld unit, but fed from a "proper" aircraft aerial, gave some very wrong information around ORTAC last week. It was difficult to ignore the nice (wrong) display and use VOR/ADF instead - even though we both knew the GPS position was suspect !!