I used it when I went over to Italy last Summer and it worked flawlessly. However, be aware that the Maps application uses a HUGE amount of data. Even though you have the International Data Plan, your still paying per MB/GB.
Also be make sure you are closing the app in the multitasking tray, it will still continue to use data if you leave it open.
As far as offline maps go, I tried a few different Apps, none were very good, and they only were available for famous cites. Like Venice (Very easy to get lost) and Florence and Rome.
Good luck to you though, have a nice trip!