In September, non stop service starts
via Porter Airlines.
Plugging in my birthday weekend YTZ->BOS and return, the base fare is $204CDN, but taxes and fees bump it up to $324CDN. Going the other way, base fare is $190US, and with taxes and fees, it's $280US.
Flying into City Centre airport, that saves me $33 round trip bus fare YYZ-downtown, or ~$70 round trip taxi fare.
Same weekend, Air Canada BOS-YYZ, it's $178 base, $265 with taxes and fees. Looks like Porter did the math as well. :)
So, future flights will probably be decided by which flights have seats available and time constraints.