Problem is that it is the content that Apple have no control. Main reason of Apple's great user experience is full control over every aspect of hardware/application. Apple should have heavily tested TomTom's map data across various country before ditching Google map. Ironically Apple is getting same criticism as Microsoft got over the year for BSOD in Windows(most of the time it was for faulty driver/hardware which Microsoft didn't control).
The fault isn;t tomtom's map data, it's all the layers on top that Apple have to add to it. Tom Tom's data (they acquired Teleatlas) is one of the two best resources (the other being Navteq) for map data. How you use the map data is the problem and Apple haven't got it right this time, I would guess most of the testing was completed in the USA, so I would expect most major cities to be fairly fine. It's once you go internationally it gets bad.
I used to work on these types of issue half a decade ago (2007), and one way of testing the accuracy is to stream GPS data in and test that it worked ok. We did this by -
a)Test programme that could load and play GPS commutes. Connect to the phone as a bluetooth GPS device. PLay the commute (from multiple countries.
b) on the phone - intercept the screen signal, using another programme, check the screen automatically to make sure the map was on the road (reading for road names and POI for more robust testing) We could run mapping/navi tests around the world without having to leave our desks.
(This testing was done for the Nokia N95/N82.. I left in 2008)
