This is something that I have been looking into as well.
I like the info you get from GPS recording to see what and where you ride.
I have used the apps on the iphone and they can range in price and function. I like anything that is free. I use all trails which does a fairly good job and can let you export the data later to upload to Singletracks. I know from my time skiing that Alpine Replay which is great for skiing is looking to get into surf and skate and might look into MTB stats as well. They are coming out with a product called Trace that is more accurate and would save battery life on the phone and work with the apps. Couple hundred bucks in addition to the phone but makes it a great GPS for cheap b/c in stead of a designated GPS unit the trace uses the phone apps to analyze the data.
As for GPS I have not used but researched a ton of them. The nice part is they are way more accurate but also loose battery. For either with a power issue (phone or GPS) there are all kinds of battery packs/solar power etc. If I were to get a GPS unit I like the watches as they have just as many features packed into a small face that could be used to replace the bike computer on the bars. Speed, vertical, etc etc can be placed in quick view.
For me the phone with a 20-30 battery pack works since I don’t have to fork out 300-500 bucks on a GPS and my phone will always come with me anyway.