For several months I've been using an iPhone 4s a friend gave me after he upgraded. I don't even have a data plan (I carry an old flip phone for voice calls). But I use the iPhone on every ride to record my routes, refer to the map and take photos and videos. If I happen to be in a restaurant with wifi I'll check into Facebook, email, etc., but usually I wait 'til I get home for that.
The freebie apps Cyclemeter and Wahoo are great for basic ride data: maps (although you can't check them in real time), speed, etc.
I downloaded the maps I need for OSMandMaps. They're stored on the phone so no cell access needed. GPS works fine with the stored maps. And there are various maps available, including some with bike routes.
A tried a couple of tricks to prolong the battery life and it seems to work. So far the battery hasn't run dead even after several hours riding and socializing.
For now I keep the phone in the handlebar bag or a pocket while I'm riding, with the apps in the background. So far I don't have any interest in monitoring my rides as I go, and I don't need turn by turn navigation. When I'm lost I pull over and check the maps. But if I was in a hurry I might use a handlebar mount for real time navigation aids.