I tried it out yesterday and the more I think about it, the more I like sites like bikely.com where you get route suggestions from other cyclists. Google was missing a couple of key trails in Des Moines. When I wanted to head downtown, it routed me down one of the busiest streets and the route was a little longer than my current route.

I expect that Google might figure out a feedback mechanism to improve their data though, so it might get better in a while.