Tomtom handhelds have it and it works great! Tomtom knows the current speed that other SIM cards in other tomtoms and similar devices are traveling. That way it knows the actual speed-driven at any moment on any street and can compute the fastest route around traffic congestion, but I am not going to put my tomtom on the window of a brand new model s! 😉
See TomTom handhelds for a couple of dollars! Could be based on Google traffic data.
The traffic feature of the current maps app is fantastic where I live. I use it every single time I am out and about to avoid congestion. And it's absolutely deadly accurate in terms of where cars are at a crawl, dead stopped, or driving smoothly.
As for the actual route navigation, it's the same as the other gps devices I have used. They all have the same errors of location, and the same instructions to turn the wrong direction. It's not just this app in this car.