I have used Google maps on Android in the US. It works very well.
As advised, it is best to download the map of the relevant area while on WiFi, and save that to the phone as an offline map. Then you can plug in your origin and destination and it will keep track of your progress (with the little blue dot and all), but it won't tell you about changing traffic conditions.
If you don't download the map as an offline map, then if it loses connection, or you have to quit the App, you may find that the detailed map data is no longer available until you can connect to the network again.
If you want live traffic data and re-routing according to heavy traffic/roadworks/accidents, then you need to have the live data feed switched on.
That can be cheap if you are on the 3 network, which offers free data roaming in the US, but can be expensive on other networks.
Check with your provider about the cost per MB while travelling in the USA.