I've taken a rental car into Canada a few times to go to Niagara Falls from BUF - never had a problem once. They ask you whose car are you driving & one time I had to show the Canadian customs agent my rental contract, but other than that, never a problem. I've only taken the Peace Bridge & Rainbow Bridge - I've never had a problem with Peace Bridge - and Rainbow Bridge can be quite a wait coming back after a weekend on a Sunday (or Monday of a 3-day weekend). BUF is a small enough airport though that it takes very little time to get from the car to the gate - especially if all you have is carryon.