Route calculation uses TomTom's IQ Routes system. Instead of just offering the fastest option based on road speed limits, or the shortest distance, the true average road speed based on historical traffic information is used. So you will potentially be suggested a different route depending on time of day or day of week, taking you away from regular traffic jams where possible. It's not an infallible system, and no match for live traffic updates (which aren't available), but still more intelligent than previous systems.
The navigation screen itself is a dead ringer for any recent TomTom sat-nav, with a clear if not enormously aesthetic map view, and an information bar along the bottom. This provides the same data whether the phone is in portrait or landscape mode, but is bigger with the latter orientation. The usual comprehensive range of details are available, including current speed versus the limit, time and distance to destination, with the distance to the next turning in the middle. The next turning is illustrated, but not the one after that as with some sat-navs. Speed camera warnings pop up whether you're breaking the limit or not, so you can make sure you remain legal.
During our testing using an iPhone 3GS, we found the TomTom software provided smooth screen updates, with clear voice commands via the car kit's built-in speaker. Overall, performance and functionality were just as good as an entry-level standalone device.
The car kit and software combination turns your iPhone into a very capable sat-nav, similar in performance and capabilities to TomTom's One and XL IQ Routes Edition models. However, the total package isn't much cheaper, either. In contrast, the software on its own is much more competitive. Navigon's MobileNavigator is marginally cheaper, but the TomTom software has a few extra features, which are worth the difference. So although the car kit and software is a comprehensive package, a generic mount and car charger with the software on its own would make better economic sense if you're only planning on occasional use.