I have used TomTom and Android phones for GPS, by far the phone kills the TomTom as it is always updated and TomTom isn't. Having said that, if you go out of range of data, the phone sucks for GPS. As far as ease of use I like the TomTom interface and flexibility for locating things (find a gas station along my route, for instance).
The iPhone is the device I would rate absolutely last as far as GPS navigation capability goes.