nokia and htc sold close to 200 million(i thought it was 100mil, but its close to 200 million) gps enable phones. nokia bought navteq for a a few billions and tomtom bought teleatlas for a few billions too. i think the number speak for themselves.
i have an htc mogul and the battery is pretty decent. talk time is about 4 hours and with the gps on it gets about 3 hours. if i'm driving long distance with the phone, i would hook it up to the power adapter.