TomTom on my 3G works fine when the iPhone is up near the windscreen, it starts to struggle however when sat down near the gear stick, where it doesn't really have line of sight to any satellites. It still works well enough for long distance navigation when it isn't next to the windscreen, but struggles in towns and other complex road layouts.

So lovely though the TomTom mount looks, I really object to paying the extra for a GPS receiver that I don't appear to need.