PS: It was probably not worth the cost:Perhaps slightly off topic, but I have to disagree - your postscript only looks at one aspect of the CA/T, there were other effects as well. For starters, the old elevated artery was way past it's useful life, and a replacement was absolutely needed. Once you realize that the elevated artery needed replacement, your options start to narrow very quickly: replacing the old elevated artery with a new elevated artery would have been more disruptive than the tunnel construction, and building a new highway in a separate location was feasible since there was no where else to put it. Digging an expensive tunnel was the least bad of a bunch of bad options, but frankly the city was paying for a bunch of stupid decisions that were made in the 1950's.
As for the positive effects of the Big Dig, it reconnected Boston with the water front, it freed up recreational land, it had a positive effect on Boston's air quality, and it significantly improved automobile traffic flow by separating out local traffic (which runs on the surface) from thru traffic (which runs in the tunnel) where before they all moved on the elevated highway which worsened congestion. Before the CA/T opened, this route:
http://maps.google.com/maps?q=woburn%20to%20braintree%20ma
would take a minimum of 90 minutes; double that estimate if you were traveling during peak hour. Triple it if there was an accident in the area. Right now, it's a 20 minute drive in non-peak hours, and well under an hour during peak times. Not bad.
That said, it would have been nice to have had more expanded transit to go along with the Big Dig. The Silver Line and the Old Colony lines (which were required as mitigation for the Big Dig) was a good start, but we could have done better.
(ftr, I worked on the Big Dig as a consultant from 1998-2001. I've also worked on several dozen domestic passenger rail programs, include T-Rex, BART Silicon Valley Extension, and the Acela mentioned in this discussion)