I agree with esh. The Droid is great as a street navigator where you are more than likely to be in reasonable cell coverage. Get outside that area though and it's nowhere near as convenient, and you suffer the drawbacks of the small screen size as well.
A little off topic, but on a recent trip I was planning to use Place Memo on my Droid as a tool to create a journal for the trip (Place Memo lets you store notes, photos and recordings associated with specific locations). The app is tightly linked with Google Maps, and wouldn't even let me save a new place if I was outside cell coverage despite the fact that the phone has GPS. IMO this is an example of a poorly written app, but it's illustrative of the kinds of hassles you can run into. By and large the people writing apps for these phones expect you to be connected to the web all the time.