Reading this topic hits close to home today as I went out for a winter ATV ride just out side of town. I was excited to get the caches that were out there located on a road where 4X4 was required. When I got to close to the place the first cache should be, I went to open the App on my phone and there was no service. Made me sure wish I had my Garmin along. So, in short, because I wanted to "Ride Light" and not carry my GPS, I rode by several caches that I was excited to get but could not.
Phone app is awesome when you just need to be close and the coverage is there to support it. I just recommend getting a GPS with the capability of paperless geocaching. I use a Garmin GPSMAP 62S and love it!