I have never taught my PPL students to use a GPS when available. That is to say, not untill right before their checkrides. I am of opinion that you should learn to be able fly without a GPS, but also, be able to use a GPS, as you should know to use all systems on the aircraft your are flying, plus it can be a handy thing to use...
I have a handheld GPS, and to be honest, i have never used it since i got my instrument rating, nor have i used panel mounted ones, except for to make GPS approaches. I found that once i really understood how to use VOR and NDB stations, it is much easier and quicker to check your approximate position.
This all said, i do think GPS will "take over" so to speak, also for PPL holders. Look at the Garmin and Avidyne panel mounted systems. But i am also old-fashioned in a way that i say "I hope everyone can still navigate their way around, when the GPS' fails"