I bought Tom Tom but never actually used it on my recent tour up the Atlantic Coast. As long as you are within 3G network access, Google Maps works very well. What's more, Google Maps will route you, as well. Frankly, Tom Tom is so oriented to driving cars that accessing the features you might find useful on a bike are kind of a bother.

If you are going to be out of 3G range much of the time, Tom Tom might be useful.

I wrote an article about using the iPhone on a bike tour that you might find interesting.

Ray