I use the TomTom app on my iPhone 4.
Works as well as the stand alone Tom Toms (which imho are the best stand alones), traffic updates, speed camera warnings, european mapping.
I bought the US and Canada version for £29.99 and used it to travel 4,500 miles around California and Nevada.
The head unit controls the iPod/iPhne re music, the audio the satnav reduces the volume on the music when giving directions then puts it back up, if a call comes in the headunit handles it over bluetooth and I still have the satnav...
And one less windscreen bracket and item to carry/hide when parking up.
Given the cost of a standalone SatNav I was happy paying £60 for the Western Europe Tom Tom app, especially as it gives me map updates, but some people use cheaper or free alternatives and say they're pretty good too!