Just back from a holiday in Italy and the roaming worked so well I just had to comment, we left data roaming on for the entire holiday, used it as we would at home (although had Wifi on the campsite), used maps/sat nav app for every trip and never got close to the data allowance. We kept track of usage in settings/mobile data/current period roaming and I hit 1.3 GB on the last day out of 3G allowance.
I managed to get Ed Sheeran tickets on the Saturday morning while sitting at a train station in Milan AND I was able to keep track of Waterfords historic win over Kilkenny on the Journal app while on the move!
Even the teenage daughter who likes to Snapchat all day was delighted with it, normally she pretends she wants McDonalds so she can get on the Wifi so we didn't have to set foot in McDonalds !!