GPS's are fantastic bit of kit and have made flying far more civilised and for those who use them inconjuction with a chart have made it safer.
I'm sure there have been plenty of people banging on about how you shouldn't rely upon them and they are absolutely right. I lost a signal in a brand new 172 in IMC on an ifr fp, luckily I was only realy using it for DME but it was part of the avionics kit ie external ariel, it was back to the old watch and crossfixes but I was a little supprised at the time.
Anyway as far as them coming down in price I have found the opposite as far as the pilotIII is concerned, they are more expensive now than when I bought mine a few years ago.
I think the real bargin is the Garmin 95 (at least I think that's the model, it's the one they have also put into a transiever). It's got all the nav functions you really need but at least £100 cheaper. I was a little miffed when a friend bought one and after using it found it to be more than adequate.
I don't know about a pilot IV but there have been atleast two models since that appear loosely based around the pilotIII both bigger and flashier but as far as I am concerned the only way I could improve mine would be a guarantee that it would never lose signal and it was idiot proof and that ain't going to happen.
They are a great trainning tool as well for the reasons already mentioned but don't let the so called purists put you off buying one, you are not cheating and if everyone had taken their attitude we'd all still be navigating by beacon fires. Just use it sensibly.