We've used our S5's this entire trip and love them. We use the Verizon $2/day Mexico plan which uses our US allocations of data, texts etc. We've had 3G/4G coverage via TELCEL, and sometimes Moviestar, for the entire trip from Monterrey to now in Patzcurao with one exception: a portion of the desert between Delores Hidalgo and GTO. Family calls and texts just as if we were in the US...all for $2/day. We even Skype'd into a wedding from SLP.
We use Google maps for navigation on the S5 almost exclusively using the Garmin as a backup...I was in the Patz big square on the phone looking for the market yesterday when your email popped up :). Maps coverage here is amazing...but it will send you down a dirt road if you aren't careful. I used it to remind me where we parked the car in GTO and route me back.
The app "SpanishD!ct" has been invaluable when we get stuck on a word...we open it and show the person the word.
The Adiante peso to dollar app has been great. We leave it open and if we cant understand the merchant we hand them the phone and they key in the price.
Bottom line is that the smart phones are very handy here. I have a cracked screen on my S5 and will be getting an S7 when we cross back!