The GPS data itself is a novelty if you don't have a cell phone signal with which to communicate to your family or the authorities.
For the last few trips, I've brought mu smartphone along. It's been off most of the time, but I usually turn it on and bookmark my location 2-3 times a day so that I can get a look at the speed of my hiking. I'll also send my coordinates to my wife, but I wouldn't rely on in a emergency situation.
On Android, I use an app called GPS Status to post links to my Facebook wall. It's simple, but it works. Here's an example from a trip last month.
I mostly hike on the Superior Hiking Trail, and I loose cell signal all the time. It's better than nothing, but if your goal is to track progress for friends and family and get help in an emergency you would be better served by a SPOT or PLB.
Battery life on an iPhone or Android phone is poor, nothing you could use on a longer trip. You'd likely need to recharge completely every 2-6 hours if doing nothing with the phone but getting your GPS coordinates and updating a map online, and I doubt that you can get a reasonably sized solar panel that'll keep your phone charged enough to pull that off on a week-long trip, even through really sunny territory.
I don't have any of the newest solar panels, but have owned a handful for charging PDAs and phones... I'm very skeptical that they could provide reliable power for a longer trip. Modern smartphones need a lot of juice.
I do often record day hikes with my phone, especially training hikes. Gives me a good idea of speed for the kind of terrain I'll encounter.