Okay - keep you hats on and sit down!
Used my regular iPhone in Italy last summer. Went to AT&T before I left to make sure it would work in Italy and the salesperson was an .... Barely answered me and simply said, yes.
It worked fine and I had no problem. But on the second to last day, I got a message about excess data usage. Didn't understand it since I had NEVER done email or websurfing.
Landed back in the US after 11 nights in Italy and took out my phone to call my kids and tell them we were home. Message now was that my service was suspended. Long story short, there is a setting you need to adjust before you go away to turn data usage off. Apparently, my phone was constantly gathering data even though I never accessed it!
Ready - I had apparently run up a bill of $1,200! I was floored and angry, especially since I had gone to the trouble of going to the AT&T store and felt the salesman should have told me about this. In the end, apparently AT&T Customer Service agreed with me. They retroactively put me on a data plan for one month and it ended up costing me under $100 - even tho it should have cost me nothing.
So yes, it works well and you don't need to do anything. But you better turn data roaming off! We went to an internet cafe each day (we were staying in a villa in Tuscany that had no internet connection) and it was dirt cheap.