I just completed a 2-day tour from San Francisco to Monterey using only my iPhone with Google Maps, the Route Planner app, and a Biologic iPhone Case mounted to my handlebar. I didn't have a map, but I figured I knew the area relatively well so if I lost the connection or battery I could at least stay close enough to reach my destination without major problems. As it was, that was never a problem during the short tour.

There was one fairly major hiccup that belies Google's bike directions "beta" status: on day two of the ride, it directed us down a road past a "dead end" sign. I figured maybe there was a path at the end, and there was: singletrack. Very narrow, very twisty, very hilly singletrack. Great on a mountain bike; not so great on a loaded touring bike. It was only a couple hundred yards, but could have been a lot worse. Totally unexpected. I let Google know about it after I got home.

The rest of their directions were great. Mind you this is in the greater Bay Area, which houses both Google and a ton of tech-savvy cyclists, so YMMV elsewhere.