Ironically, the places you need GPS the most are the places there is no cell phone coverage. As much as I like my Android its my Garmin that goes into the backpack.
In other words, the way you personally use your GPS device must be the way everyone else does, right?
You talk about putting your Garmin into your "backpack" and using it where there is no cell phone service. That's sounding to me like you're taking it hiking out in the back country. Which is fine, but that is not what most people use GPS for these days, nor is it what Google Navigation is intended for. It's intended for use while driving. (Indeed, many GPS devices produced these days will not work when off-road.)
Most places in the United States where there's a road, there's cell phone service. No, maybe not on some rural route in Idaho, but certainly in the most populated areas. And while it may seem counterintuitive, it is actually more helpful to have GPS for car navigation in the most populated areas than it is in the least, the reason being that there are so many more roads, which means so many more turns. To get from my store in Manhattan to my home just outside the city - a distance of approximately 14 miles - requires something like 45 different maneuvers and the use of about that many different roads.
Now, the REAL killer app for Google Navigation, which will be apparent to all eventually if it isn't now, is free cloud-provided live traffic. Most current GPS devices that provide traffic info (and remember, they all force you to pay for it in some way) do it the old fashioned way, usually by subscribing to a service that's taking call-in reports from local police or utilities, or even individual commuters. This info is always old and often wrong. Google Maps' traffic is live, taken from the cloud. Right now, my wife and I have gotten into the habit of having our GPS hooked up and having one of our phones out with Google Maps loaded up to check traffic on our route. (Remember, this is New York.) And it's always right, but there's currently no easy way for us to do anything about it when our GPS device guides us into a "red" traffic area. (We can press the "detour" button, but that doesn't really guide us around traffic, just a pre-set distance.)
It's going to be amazing having free live traffic data integrated into Google Navigation. The only thing I haven't seen is whether there's a way to tell the app to "avoid traffic" when constructing a route, or to "detour around traffic" if traffic develops along the way. But that should be pretty easy to add if they haven't already; just another little algorithm.
And that's the *other* great thing about this - free updates. I had to pay $80 for map and interface upgrades to my Magellan Roadmate 2200T, and while it was worth it, they only ever produced that one update and I sure would have liked it to be free. Especially considering that the update itself has its own problems, which I have now just had to live with - for example, it now messes up the side of the street destinations are on about half the time. No way to fix this except to buy a new device with new software on it. It also constantly drives me into a dead end when I go to my mother's house - the map is out of date. Again, no more updates are coming - gotta buy a new device. Waste of money.