Accuracy is subjective to location, in Atlanta...it's flawless, driving outside of Atlanta (from the border of Tennessee to the border of Florida) it's still flawless.
When i visited Boston last Christmas, however, it was not as useful...in the metro area it was fine, but when we got closer to New Hampshire, it started acting wonky, giving us wrong directions or missing shorter routes. I don't specifically blame Apple maps for this though, because when we attempted to use the rental cars navigation system, it was also not flawless, and I'm guessing it has to do with the mapping and verification of routes in the area...if it's half assed done, you're going to get half assed performance, regardless of device.
Apple maps has been fantastic for me so far, so good in fact that when the google maps crazy went into over drive and the app was released for the iPhone earlier this year, I didn't even look at it...no reason to, I'd rather use my native app that works perfectly for me.