My experience relates only to EU travels, with the last trip in August this year.
Five years ago I've rented a car in UK and was not asked for an IDP (nor did I have one), but last summer in Athens, the car rental company specifically asked me for the IDP (and I had it this time). Read an angry review later about somebody complaining how the same company denied him a rental because he did not have an IDP.
Canada and US did not sign some obscure treaty, and therefore US and Canadian DL holders require an IDP to drive in EU. It appears that the level of enforcement for this requirement went up a notch this year.
Bottom line, if you'd like to avoid the hassle while driving in EU, or you are planning to rent a car there, it's worth getting an IDP. As others have stated, it costs only $25, mine was ready in 10 minutes and is valid for 1 year.
The identity of Ireland's most notorious motorist has been uncovered, after Police realised they were recording Prawo Jazdy – the Polish term for driving licence – instead of the offender's name for more than 50 different Poles.
Greece needed one. Similar experience as someone mentioned, rental agencies like that I had it, but mentioned for people without they just warn them not to get caught or tell the police they left it at the hotel.
Was never stopped but $25 for piece of mind? Worth it IMO