I agree, though I'd also point out that if there's anywhere in North America with a large population of people who don't own cars, it's New York City (and to a lesser degree Montreal), so when you have to rent a car that changes the driving vs. flying cost and time equation quite a bit. (Many times the easiest place to rent a car is the airport anyway, so you're making that trip already.)

That said, I did just get back from a long weekend road trip to Montreal and I wanted to put in a plug for renting from the Hertz at the North White Plains train station -- it's right within the commuter parking lot and the 40-minute train ride from Grand Central (on express trains, it's about 1 hr on local trains) gets you far enough north to be out of the worst of NYC city traffic.