Today Google Maps Navigation comes to the UK and Ireland, and will be available as part of Google Maps on any phone running Android 1.6 and later.
Even if you haven't used a satnav tool before, you'll know your way around the Google Maps interface.
My experience using the TomTom iPhone app hadn't been that compelling; it was occasionally unreliable and often failed to find a signal, even outdoors with clear skies. You always had to enter a street number - or make one up if you didn't need one address in particular. It does include a basic Google search and live-ish traffic info has always been a strong point. But can Google Maps Navigate do any better?