The first is that when I use SatNav I arrive at my destination having no real idea where I am. I remember in a science fiction book reading a statement like "We claim to have explored space, but in truth, we have explored the small bubbles that surround individual planets. Of the great depths of true space we have no knowledge." I feel a little like that now when driving longer distances with the SatNav.
Secondly, it's interesting how often the SatNav produces "obviously" sub-optimal routes. The Travelling Salesman Problem is well-known to be NP-Complete, so it's not surprising, but some of the foibles are incomprehensible. It's no wonder some people distrust them, and quite surprising that others seem to have complete faith in them.
Finally, stop for a moment to consider the miracle that is the modern SatNav. Despite its short-comings it contains a device to tell you where you are to within a fathom, maps that know where you are and where you're going, enough power to compute a sensible (if not always optimal!) route between the two, a suer interface that non-technical people can usually drive, and all for the cost of a few hours work.
Addendum: GPS works by computing varied flight times of radio pulses. Light travels about a foot per nano-second, so a GPS receiver has to work with time differences less than a typical clock cycle (for the on-board computer). If you have a 1GHz computer, light cannot travel across the motherboard in a single clock-cycle.
Stunning.
PS: Yes, I do know the details, and I know the inaccuracies in the above.