Yes, the smartphone apps like Google Maps are getting better at downloading data ahead of time but, if you need to detour, you may not get the smaller road detail you'd already have with a dedicated GPS device.
Personally I'm using Google Maps on my Android and find it works well enough. I also use a Beeline which is a round 1.5" diameter route indicator device that mounts to your dash or handle bars. It Bluetooths to my phone which is safety tucked away in the left storage bin. The screen works GREAT in full sun and gives just enough info at safe quick glance to navigate your per-set route.