I have a bluetooth GPS receiver, but I don't use TomTom or any other expensive GPS software on my Treo. I do have some older Mapopolis maps from when I had a subscription a few years ago, but they don't work very well on my Treo 680 like they did on my T3 or Zodiac2. What I have been using is Google Maps and the "dontationware" Where Am I? application. It doesn't give you real time location while you are driving like TomTom does, but it is a very good alternative. BT GPS receivers have really come down in price over the last couple of years. 
An example of my typical usage came yesterday when we were going to a going away party for some friends that are moving to Colorado next week. We were running late and had just left the house when I asked my wife if she remembered how to get to the party. She didn't remember and I didn't remember. While I drove, I told her how to use the BT GPS that we keep in the car and the WhereAmI? app to get our current location in Google Maps. After that, it is just a matter of plugging the destination address, which we did have, into the directions screen and we have directions from our location on the highway to the party! There are times when I have considered spending the money on the latest TomTom software, but I really don't need it.