I sold my Garmin GPS (used it for driving in Europe twice) once I realized how much more advanced my phone with Google Maps would be for driving.
You don't even need mobile data on your smart phone to use it as a GPS. If you use Google Maps, you can download "offline areas" ahead of time on WiFi, the put the phone in airplane mode and drive with no internet. (There are other mapping/GPS apps besides Google Maps. Everyone seems to have a favorite.) I used my phone this way to drive in Slovenia earlier this year. Worked surprisingly well. But it's not perfect - the version of Google Maps with mobile data turned on is a little better (gives you traffic info, etc.).
If you've never used your phone as a GPS at home, try it: download the "offline area" for the area around your home, put the phone in airplane mode, and tell it to navigate somewhere nearby and go for a spin. If you're satisfied with that, it will work about the same in Europe (except everything will be in kilometers if you live in the US and are used to miles).
Even though I had my phone in airplane mode most of the time while driving in Slovenia, I had cheap data on my phone so probably didn't need to. I had an unlocked Android phone and had bought a Dutch Vodafone SIM on eBay before my trip - total cost was $30 USD including 2 GB of data good for 30 days. Free roaming in the EU since the EU has done away with most roaming fees. If you live in the US and have T-Mobile or Sprint, you already get free international roaming data on your phone, anyway. AT&T and Verizon charge $10 per day.
