Off-the-shelf stuff is attractive, at first glance, as it tends to be MUCH
cheaper and lighter than mil-spec equipment.

But the stuff at CompUSA wll tend to degrade and eventually die when
operated under your typical harsh mil conditions.
Your typical Dell laptop will eventually fail when subjected to the
temperature extremes, vibration, bumps, cable tugs, sand and dust that
equipment encounters in the field.

The mil-spec equipment will instead have 1/4 inch thick aluminum or
tuitanium cases, cables and cable connectors that you can tow a jeep with,
and lots of o-ring seals around every sliding or rotating part to keep out
sand, dust, and water.

So the $150 GPS's your mom sent will be wonderful to use for a while, but
after it has been dropped 20 times, onto rocks, sand, water, had 9mm rounds
bounce off it, you might want to pull out the clunky old mil-spec GPS, which
will still work after all that.