I can't speak for Canada, but lots of ISPs in the US started this. Ours is 250GB a month and we've never hit it. I have a niece that does nothing but stream Netflix all day long -- and she's never hit it either.
I can also tell you the number of folks that hit it are really low. It's intended to send away the 1% of users that uses 80% of the bandwidth.
I do, however, really dislike the concept. It seems wrong to bill for something you may or may not have control over. For example, a couple of days of denial of service attack WILL trigger the bill. I suspect as this starts to happen, ISPs are going to have to rethink the policy.