Toronto has a secret that it doesn't advertise (read: I didn't see it, but given that I did not research before going that doesn't actually mean a whole lot) that is probably one of the best features of Toronto International... If you're flying to the USA from Toronto you do all the immigration and customs in Toronto - you just have to collect your luggage when you get to the USA. Given that my recent entry via Dulles gave me a 2 hour wait at immigration (and for a 8 hour flight that's an extra 25%, in addition to the 4.5 delay in departing LHR) doing it here is a welcome relief and means I'm far more likely to route via here if a direct option isn't available (gives me an unneeded excuse to visit Toronto).
Oh, and you can take the tube + bus to/from the airport too. Congratulations, you've just saved $60 on a cab fare.