I have flown it several times. Weather has always made our decision as to which way we went. It's a definite cost saving to crossing the border, but then you can't say you flew across Canada

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We have had to fly north of lake Nipigeon once and even Valdor just to get home. It seams that everything comes in up the coast and from the great lakes area and hits the maritimes all at once. Each time we planned to head south to New York we had to actually go Northern Quebec to get around the weather. Valdor is NOT New York

(Guess its not really northern Quebec but it felt it)
We like Moncton to Carp, then on to Sault St Marie US side for customs. Then take the south shore of Lake Superior. We found it the quickest for a bit more civilization. Be prepared for long periods of radio silence if you are in the woods of Quebec and Ontario, but that is the shortest route overall.
You have to fly the Parry sound area. It was the highlight of all the flying we did down that way.