I've joined in thread after thread here about whether or not to buy an IDP before renting a car in Europe. I take the side that there's no need to bother for western Europe (can't speak for farther east). Car rental agencies don't ask for one. Police at checkpoints or at traffic stops have been satisfied with my state's drivers license.
Others here, quite logically, believe that some countries, particularly Italy, require an IDP and advise "better safe than sorry". After all, they're easy to get at AAA and cost only about $15 plus the cost of a photo.
My question is this. Has anyone had first-hand experience of being penalized for not having an IDP in western Europe?