As you explore you'll want local maps on your iPhone but if you are visiting the states from another country you could easily run up substantial roaming data charges. With this app, you will avoid them. A map of NYC is stored directly on your device. Personalised markers allow you to annotate the map according to your needs and when you have access to data (hotel wi-fi for example) you can use iOS location search and store the results for offline use.
You needn't worry about separately downloading map data - it's all taken care of within the app and yet it manages to use only 54 MB of storage. Better still, it's free for basic use and a small extra charge allows you to see extra mapping detail and customise markers.
The map data is sourced from OpenStreetMap. This is a freely editable map of the whole world and is more up-to-date and detailed than its competitors.