This is one of the best brewery tours I've ever been on. My thoughts on reasons why this may be...
1. All of the tour guides are slightly buzzed
2. They get free beer every day to take home
3. The unique culture and tradition of Boston
The tour guides are wonderful, and you even get a free glass to take home at the end of the tour.
If you go around St. Patrick's Day, the lines may be a little longer, but you'll get to sample the 26.2 Brew that's made exclusively for the Boston Marathon.
It's my favorite.