In my experience Google Maps is very accurate, but the turn-by-turn instructions are generated by the built-in navigation - and its maps are not nearly as up-to-date. This leads to bizarre situations where the turn-by-turn leads you completely astray, yet if you ignore navigation and simply follow the map it will lead you to the exact right place!
On a recent trip to Montreal I was trying to get on a highway and it was telling me to drive in circles. On my previous trip I was trying to get to a mailbox company in an old industrial park in Ogdensburg NY, and it took me to a children's mental hospital over a mile away. (I think it was trying to tell me I was crazy for following navigation.) It's odd because in both of these situations the roads in question were not newly built.
All that said, it works perfectly most of the time. But when it fails, it really fails.