HOoray is the right word to use as an unlocked smartphone with a Spanish SIM card will unlock enough Apps to assist you to fully savor the 21st Century Camino. There are many things you can do without an internet connection apart from phoning, inter alia, open the eBook to look at your library of places to stay, addresses, route distances, maps which you can zoom with your fingers, etc. Write notes, copy and transfer pictures from your camera card (buy the SD Card attachment separately), and more. With a wireless or bluetooth internet connection you need not hog the computers to check mail, speak to home anywhere in the world free on Skype or Viber or other multitude free VOIP phones such as Freetime, use voice memo to record notes for the day for later transcribing, check weather, dictionary and translator, convert units and currency, see news, check flights,etc. There are excellent finger drawing Apps so you can sketch to add separate illustrations, in addition to any photos you have taken. I do not wish to info overload you.
You should take into account that you can do all that on a data plan if you do not have an internet connection, you will be able to connect to the internet though 3G or 4G. It will as Evanlow says cost a bomb to use, and to be used only in an emergency, make sure the 3G is not switched on automatically.
The other thing to watch out is that the iPhone is power hungry, use it sparingly with minimal Apps, switch it off completely when not in use. Consider carrying a spare power pack which you can charge and use outside the iPhone, it keeps the worry of guarding a charging iPhone. Or a solar trickle charger on your backpack.
Evan Low on this forum is a tech guru and it will be exciting to read of his adventure, particularly the tech geek side of his Camino, when he in on the Camino in May/June/July. It is a relief to see that those with tech phobia getting less strident in this forum condemning the use of modern equipment in the Camino. It is a personal Camino for everyone and I wish you and Evan Low a Buen Camino.
I too am clicking my heels and doing the dance for our fellow smartphone, iPod and iPad users on the Camino.
Joe