Having a smartphone means you can do a myriad of awesome things. It can look up restaurants, connect you to social media, and navigate from point A to point B. This makes it seem a bit silly to get a completely different gizmo to use for navigation if you already have a perfectly capable device.
If you want to use your phone instead of a Garmin or TomTom, then why not get something like the ARKON Windshield & Dash Smartphone Mount? It can stick to the glass on your windshield or on your dashboard, and will hold just about any smartphone you put in it. It can be in landscape or portrait mode, and rotates 360 degrees. They made sure to give you an unobstructed view for your camera, though it probably wouldn’t be a grand idea to take pictures on the road. This will cost you around $11, and would be rather useful for taking calls on speakerphone as well.