GPS: It's indispensable if your trip takes you off trail into the open backcountry. However, you still will want to carry a compass and map, because only the common 1:24,000-scale topographic map can adequately detail the terrain and give you the reassuring overview that you are, in fact, heading in the right direction. The device I took, the Garmin Colorado 400t, consumed a lot of power, so I had to switch it off frequently. You can bring extra batteries, but that points up a major downside of backcountry electronics: weight. Every ounce of battery and circuit means a tradeoff with some other useful piece of gear.
PLBs: Personal locator beacons are not quite as empowering as they might seem. Yes, they are the ultimate safety net, so if you find yourself, as they say, having "exhausted all other means of self-rescue," you can press the button and government — park rangers, police, Coast Guard, whoever — will come running. But emergencies are not all equal. In my case, I easily could have been immobilized by my feet, but a rescue helo would have been out of proportion. Also, if the search-and-rescue services determine a call is frivolous or the result of poor judgment, you can be charged for the rescue.
Satellite phone: This is an enormously useful device that allows you to update folks back home of your progress but also to reassure yourself that everything's cool at home. Surprisingly, this was the most valuable function of the Iridium sat-phone, allowing me to stay out several days — including the two days necessary to place my water caches throughout the park — without fretting that home life was falling apart. Also, a sat-phone allows you to explain the nature of an emergency. Perhaps you don't need a helicopter rescue. Maybe you just need a ranger to bring you some water. In any event, renting a sat-phone seems like an excellent investment.