When researching our trip to France and Italy this year, we weren't going to get an IDL until we came across some driving info on the Slowtrav site. It seemed to indicate that you'd likely need one if you get stopped in Italy but less so in France. We picked one up for our trip but never have to use it. We weren't stopped by the cops and the Hertz rental places didn't bother to look at it.

If I had to do it again, I'd probably get one if I drove in Italy again. The driving out there is pretty hectic so I'd think the odds of me encountering a situation where I'd need to show it would be higher than other countries. Plus, we seemed to run into such wacky rules and inconsistent rules out there (not necessarily driving related), I wouldn't doubt that we could get seriously nailed for not having one.

[And yes, as previous posters have mentioned, IDL not valid without your regular driver's license so it's value alone as a piece of ID is pretty limited.]