It cost a ton more than any other way of delivering a similiar result. Operating costs are higher than a raised highway because it's underground. The only pro is that a bunch of rich progressives who don't actually live in Boston get to pat themselves on the back about how the old eyesore of a raised highway that "divides the city" (or some mumbo jumbo like that) is gone. That's not enough to be worth the price. It was a boondoggle. They should have just rebuilt the raised highway.