GPS unit or iPhone GPS?

Emily, It would probably be less expensive to use a GPS unit. If your iPhone accesses the data network at all, the charges for data roaming will be extreme! You can reduce these to some extent with a data roaming plan. I assume you're with AT&T, so it would be a good idea to contact them for specifics on the rates. Cheers!

If you're driving, and if you'll be bringing a laptop, you might consider buying a Holux (or similar) GPS and a copy of MS Autotoute. Or, get the Autoroute version that comes with a GPS receiver. Autoroute offers the ability to plan and evaluate different routes, and import data like campground addresses or Carrefour store locations, or...

You can buy a stand alone GPS unit that has US and European Maps. You can also add European maps to most Tom Tom, Garmin, Magellan etc units. If you want to use your iPhone on the road you need to get an app that has maps loaded i.e. Tom Tom. The standard iPhone maps load as you need them from the internet so using that in Europe would require an internet connection which can be hard while traveling as the US iPhone is locked to AT&T so you will be roaming or downloading over wifi. http://itunes.apple.com/us/app/tomtom-western-europe/id326075062?mt=8

A good GPS unit provides an easy to read screen as well as spoken directions and notifications of upcoming route points. It will also notify of fixed speed camera locations. I've read that TomTom has an iPhone app which doesn't require a data feed (maps are built in), so that's a possibility if it does all that. We have a Garmin GPS unit which worked very well in France last summer. Regardless of whether you take it for GPS purposes, the iPhone is a handy travel device. With a data roaming plan, it's possible to manage data usage and be able to utilize the phone at a reasonable cost. It's useful for getting information and addresses and opening/closing times of museums, restaurants, etc. With the wifi feature, checking email and using Skype is easy.