The one thing that was most awesome about every Casio G-Shock/Pro-Trek I ever owned was the solar power -- never needed a replacement battery.
Now bear with me here because I know the Fenix line is a totally different kind of watch that uses significantly more power, however, given the 20/72 hr limitation when being used in GPS/Ultratrac (when you're likely to be outside getting sun), has there been any investigation into how much adding solar juicing might increase battery life and make charging less frequent? This would be an unmatched capability for those who backpack too, to be able to charge it up with the sun between power sources.
I just bought the 5X last month, and obviously it's no contest between a Pro-Trek and a Fenix, they're not in the same league, but I miss the crap out of my solar powered device. It seems like the panels can be pretty compact based on how little space is taken up by panels on the Casio line. It wouldn't be effort-free, but it should be doable. Dunno whether others have thought of/requested this, but this is the one thing the watch lacks as an ultimate outdoorsman's tool.