I ride with an iPhone tucked into my handlebar bag.
I pull it out once in a while to check my route on google maps, especially if I'm on an unfamiliar country road. In this part of Central Texas there are so many winding country roads that don't necessarily go where you think they might because there are few bridges over creeks and rivers.
The iPhone works great as a reference map if you just need to check your route. But if you ride out of cell phone coverage you are lost.
I'd like to get one of the new mapping Garmin edge models but can't really justify it. a traditional cateye bike computer plus the iPhone is really all I ever need around here. especially since I'm going to be carrying the phone anyway