I use the nav all the time, and rarely have a of problem. I'll do a quick review of the route first just to make sure it seems sane. For local roads, it sometimes chooses an odd route, but I just go the way I think I should go and it corrects itself en route. Often when it switches to "my" route, it's immediately all-around better (faster, short, less power).
For long trips, SC to SC, it's been nearly flawless. However, in those cases I check first Google maps to find the optimal route if SC stops were not involved. Then I check with both A Better Route Planner and EV Trip Planner to see how they compare to optimal. Those two sites often show different routes, both of which are worse than I can do myself. The in-car nav usually agrees with what I think is the best route.