I use Google Maps alongside with USGS and USFS maps when I plan a Jeep junket. I just find it easier and faster to browse Google Maps than an actual topo map. The downside is that Google Maps isn’t always accurate. I’ve wound up facing dead ends a number of times following roads that appeared to connect on Google. Still, Google is great for general reconnaisance.
Now Google Maps gets an extra boost of goodness from the Terrain overlay. Clicking the Terrain button gives a topographic view of your standard Google Map and it gives a better understanding of the area you’ll be traveling. Again I’d use this just to get a general idea of the lay of the land, using a USGS map for confirmation. Specific elevations aren’t marked in Google and the Terrain feature doesn’t work very well when you’re zoomed in. But it gives a nice birds-eye view.