We spent tons of time researching phones/phone plans/sims/etc for our trip and even brought an old 5S with us for potential purchase of a SIM card here in Europe. A friend told us about a new international plan that ATT offers and I'm not sure how new it is but it offered all data/calls from our existing IPhone for $10 per day. It's not available in all countries but we are mostly just in Italy so it's available there. You can turn in on in the states prior to your departure and it's only active on days you connect overseas via roaming. So, if you leave the phone on airplane mode for the entire day, there is no $10 charge. If you are in the US there is no $10 charge (we were told but we went ahead and waited until the morning we left to activate).
It's certainly not the least expensive option but it was very convenient to keep up with family while we were on the road as we got to keep our number. I was even able to use the data over a wifi hotspot setup on the device. I'll update once we see the bill for this if there are any differences but wanted to put this out there in case anyone had used it or knew of this option or perhaps someone was interested in this.