Yes, we bought the GO card and got our money's worth. We were there for a week. Let me see if I can remember everything we did with the GO card. Oh here is the place we purchased them for a discount:
http://www.smartdestinations.com/boston-attractions-and-tours/_d_Bos-p1.html?pass=Bos_Prod_Go
Tour of Fenway - a big highlight for the boys, and I enjoyed it too! We did take in a game too which was huge for my boys. Have to be careful of obstructed view seats.
Duck Tours - second favorite. GO card only worked with Museum of Science dpearture point for Duck Tour. Then we did the MOS - both the Duck Tour and MOS were on our GO card.
Freedom Trail - we used the tour guide I think was on the GO card. Another Great decision as this guy (dressed in period costume) was so hysterical and entertaining that my then 11 year old actually loved the Freedom Trail. If we had done it on our own, he would have been bored.
Aquarium - GO Card -a bit small, but loved it. We also hopped on a harbor crusie which we didn't plan on, but was on the GO card so we did it.
Mansions in Newport RI. THank God I remembered the GO card because we were there for a beach day, and it poured rain. Used the GO card for tours of the mansions.
Spent a day in Lexington/Concord - used the Liberty Trolley - great. GO Card again.
As a side trip, we rented a car (one day for RI and one day for this) to go to Maine. Beach day at a bit north of Old Orchard Beach, and dinner at the Lobster Shack on Cape Elizabeth -that was for me!

No Go card that day.
Could have taken the ferry to P-town on the GO card, but just didn't have the time. We need to go back to go to the Cape!!
It really was the best vacation. We walked around North end and ate at Regina's pizza and went to Mike's Pastry. I researched the heck out of it on tripadvisor, and hotwired the Westin Back Bay for a great rate. I think I got the GO card for 5 days, as we didn't use it day #1 or #7 when we left.