Use it all the time, I have not fired up my TomTom in a while! Have the Destinations app on my iPhone with allot of my saved points.
The only down side to use this is if you go off the downloaded route. It does not automatically re-calculate the route and display it like a regular GPS does.
In this case Sync tells you that you have left the planned route and asks you if you want to update it. Then you have to go through the whole download process again. It is not the fastest process....but it works.
The only issue I have is once and a while I had problems downloading the route. It would fail. This is not a Sync issue I realize....more of the cell phone the signal I am getting. I now have the iPhone 5 so hopefully with LTE/4G speeds I will not have those issues again.