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)

Releasing once a year and making improvements over time is definitely the modus operandi at Apple when it comes to iOS software and devices.