I've used the tomtom car kit with an iphone 3g for a year and a half, and the gps reception was much better with the car kit, making car navigation really quite functional; driving through a skyscraper canyon in a major city it gets iffy, but otherwise, it's been pretty good. Just upgraded to the iphone 4, and for the record, it has worked fine for me with the TomTom car kit so far. There's some debate about whether the car kit gps receiver/antenna or the iphone 4s' gps receiver is more accurate; the consensus is that they're close, in any case. (I think one of the major factors in the accuracy of gps reception is antenna size and design, and the iphone 4's use of the metal band as an antenna might have something to do with its improved gps accuracy.)
So, if I didn't already own the TomTom car kit, I'd be looking at alternatives like the Kensington 2-in-1 Car Charger and AUX Audio Cable coupled with perhaps a Proclip passive mount, which would give a solid mount plus charging plus aux-out to play things through my car stereo. Wouldn't have the TomTom's speakerphone, but some may not be interested in that part anyway.
So, if I didn't already own the TomTom car kit, I'd be looking at alternatives like the Kensington 2-in-1 Car Charger and AUX Audio Cable coupled with perhaps a Proclip passive mount, which would give a solid mount plus charging plus aux-out to play things through my car stereo. Wouldn't have the TomTom's speakerphone, but some may not be interested in that part anyway.