My wife has a TomTom Go730 and I have a Garmin Nuvi 760.
My wife thinks the TomTom is great except for the Phone bluetooth not going through the car radio. She bags my Garmin every chance she gets especially the 'recalculating' and 'drive to highlighted route' instructions it gives.
I think the Garmin is way better than the Tomtom but neither of them is perfect.
I think the Garmin has a better music player and the fact that all audio including the bluetooth phone can be directed through the car radio is a sure winner. The Garmin also does voice dial and the phone's individual caller's ring tone comes through the radio.
The TomTom is much more customisable but the bluetooth phone audio through the internal speaker is a show stopper. Also have to select from the phone book or enter a number rather then true voice dial is a let down.