Just went through the same decision process as I will drive in Europe on 3 trips this year. I am good with directions, and on past trips google maps actually did pretty well the few times I was lost (and I do not pay for data). But will be in a few pretty remote places and wanted something more robust. Also did not want to purchase a dedicated GPS even though you can find several good deals on eBay.

In researching I found that the TomTom and Navigon Apps iPhone apps do not require data access. The maps are stored on the device and it can take advantage of the assisted GPS capability of my iPhone4. TomTom just dropped their full Europe app to $99.99, and then I was able to purchase $100 in iTunes credits for $80 during one of the black friday deals. Of course then Navigon dropped their price even more for the holidays - $69.99.

I choose the TomTom app when they were priced the same. Garmin has acquired Navigon, and already it looks like they are adding costs for updates. But if it had been the lower price when I decided, I may have chosen Navigon as I think I like its look and feel better.

My first trip is next week and am going to try using the TomTom app on my iPhone. Bought a $5 windshield mount to keep it visible. If the screen ends up being too small, I will probably invest in a Bad Elf GPS receiver and use that with my first generation iPad for the future trips.