Just spent two weeks in Italy and TMO was pretty good. Only complaint I had was in the last two days I was there, my phone (new Note 4) seemed to have a hard time getting a data connection. Odd that it was fine for the first 10 days (no problems at all) but for some reason very balky at the end. I was in different places so that may be part of the problem. OTOH, my wife's Nexus 7 kept its data connection up more reliably.
Last winter, when I was in Sicily, had no problems at all provided there was cell coverage.