I use a phone rather than a dedicated GPS. However, you need to plan for the deficiencies of a phone, which in my opinion are:
- Poor battery life. This can be mitigated by keeping it in airplane mode, and not tracking your location continuously - ie, use the location services only when required. You can also carry an extra battery (which can plug in to the usual charging port) if you think you need it (eg, for long multi-day trips).
- Easily damaged. Recent models are often waterproof, but they are still easily damaged by dropping, knocking, etc. You need to either be very careful, or use a rugged case (or both).
If you are OK with these two limitations, then I reckon a smart phone is a LOT better than a dedicated GPS. You can do so much more with a smart phone, and it does all of the GPS stuff better than the GPS - it just depends on having the app(s) you require, and the map(s) that you require downloaded.
I have never owned a dedicated GPS, but when watching other people use them, it amazes me how terrible the maps are, how tiny the screens are, how poor the image quality is and how woefully terrible the user interfaces are. The GPSs may have improved, or perhaps I've only seen poor examples, but from what I can tell, phone apps are vastly superior - so long as you download the app(s) that meet your requirements.
(I can't recommend a particular app - I use an iPhone app that I developed for myself and it is not available on the App Store.)