This is my biggest complaint with Google Nav.
Navigation will get you there, and if you have no clue where you are going it's a huge help, but I have tested it on my commute to work and home. It's highly inaccurate on my drive home. It spends half the drive trying to force me to an expressway which is red / black.
Sitting at a stop light about 1/3 of my way home while sitting at a stop light Nav was telling me to make a right as it attempted to force me onto the Eisenhower. Nav showed 54 minutes to go on my commute. I made a left at the light like I always do (yes 180 degrees opposite how it wanted me to go) and right after the turn Nav's time estimate dropped to 28 minutes.
Nav is nice, but it has a way to go to compete head to head with other Nav systems. That said it's the only Nav system I own.