There were multiple problems along the way. It ultimately ended up costing $22 billion. People were also upset about this.
At the end of the day? It was an awesome project. They completely buried a highway that cut through the city, and added a ton of green space downtown (The Greenway).
The city is better for it. Taxpayers would never have allowed it if they had their way.
We need a similar program to revitalize the MBTA. It should be done regardless of the cost. I will happily pay more real estate taxes and highway tolls for the rest of my life to support it.