Have a look at the last post on this thread Google support forum. Based on that the 1 Gb is insufficient. The problem is that the map isn't stored on the phone and has to be downloaded. There are other apps that store the map.
I did a comparison using Google Navigator on my smartphone and my TomTom. I found that the satnav on the phone wasn't as accurate and the voice was irritating. If you lose signal then you're stuck. Street view just as you approach your destination is fun though.
A rival mag in their July 2011 edition did a review comparing stand-alone satnav and satnav apps. Their recommendation was for a TomTom standalone. For iPhone they recommended TomTom app and for Android CoPilot Live 8 but neither are free. CoPilot Live stores maps on the phone.