Yes, smart phones batteries are not long lasting, that is their Achilles' heel. You can however buy back-up batteries pretty cheap that should give you enough juice to last the ride. So far I've never tried to use the Navigating function in an outlaying area where phone service is poor or none. When ever I go anywhere in the car I now use only the Android for navigation whereas I used to use a Garmin Nuvi. The Google Navigator is much better than my Garmin ( Nuvi ) although it does lack some of the special features of the Garmin.

For bike use the Android has it's limitations but only because the current offerings of Apps are lacking in sophistication. I've tried the "Cue Sheet" app and it does work but as you mentioned does not give turn by turn ( audio or visual ) prompts. The "Cue Sheet" app is also "slow" in keeping with your progress via the Phone GPS. Google navigator on the other hand does not have that problem. I'm hoping that someone will create an app that will merge the "Cue Sheet" app with Google Navigator. Done right it would give all the mapping/Cue sheet functions of the "cue sheet" app but give you the fast tracking and turn by turn announcements of Google Navigator.

For now the Garmin's ( 705, 800, 810 ) are the best for bikes because they run longer, store more maps and offer the bells and whistles most people seem to want. Hopefully if better apps come along for the Smart phones that might not be for too long.