There are two parts to the equation, the hardware and the user interface.
As for h/w, I love using my small tablet (Nexus 7, 2nd edition). Small enough to carry somewhat comfortably in a pocket, but larger than a phone screen (at least for now). My middle aged eyes do wish for a larger screen sometimes. I have a RAM Mount tablet holder mounted in three of my vehicles, and just move the tablet. One note, some of the cheaper tablets do not have their own GPS systems, and instead rely on cell phone tower triangulation. Won't do you any good if you travel where there are no cell signals.
As for the User Interface, I've been using BackCountry Navigator, though they are close to releasing their next product, BackCountry Navigator XE, funded via KickStarter. A very powerful GPS/mapping system, but somewhat challenging to learn how to use to its fullest extent.
At least for paved road driving thought, there's a lot to be said for the dedicated windshield mounted systems.