I also use a Garmin eTrex and it helpful, but I would never substitute it for a compass. More important than either for outdoor navigation is a good quality topographic map; only it can inform you of the cliff or deep canyon that lies between your objective and your present position.

Even with extra batteries, a GPS has a finite life (about one day per set of batteries for an eTrex if it left on for normal periods). My compass will go on performing, with no spare anythings, long after I am not. They are simply more reliable and foolproof than anything electronic. Or let's put it this way - which could be pulled out a PSK and perform reliably after being packed away for about two years - a compass or a GPS?

About ten years ago, I was using an early model Magellan, and following the instructions, I had packed along a spare set of battereis in a spare carrier (the unit would lose all settings and memory once it had been deprived of battery power for longer than two minutes). Working along, I got the message on the GPS screen - "replace batteries or lose data" I quickly and smoothly inserted the fresh carrier and turned the GPS back on, only to see a blank screen.. Fortunately, I had been hand recording readings as I acquired them. Murphy's Law does apply to GPS...