Char, before you invest, check your phone & contract, you may already have it available to you. I've never used it, but on my current carrier and the phone I have I can subscribe to theirs for only a few more bucks a month. Unless it has changed, purchasing the TomTom unit is a separate expense, you also have to subscribe to the service to make it work.

I don't use it because to me, it's for wimps.

ETA: I just checked, it's $2.99 for single-day use or $9.99/unlimited a month and it works with many phones. My carrier is Alltel.