On a recent bike trip around a new city Google Maps was a godsend, but the navigation feature had to sync to 3G every step of the way, sometimes leading to annoying lag times. Other times the feature flat-out didn't work.
Right now, with caching inside of Maps, you can slip outside of a coverage area and still get navigation directions for quite some time, but you’re SOL if you leave the area that’s cached and still can’t seem to grab a connection.
Best of all: the app is still free. I can't imagine a navigation competitor making a dent in the market now..