I had my ducts cleaned by an east Indian guy from the GTA (I'm in Sudbury) this summer, did a great job.
He opened up the furnace to clean the heat exchanger and fan.
Did my A.C. coil as well.
Very profesional.
BTW the only way one of these guys can rest tag your furnace is if the have a G1 license. On the tag it must list what is wrong, their name and License number.
Most of them can't speak enough English to pass the exam.