With 4th Of July, they have a couple hundred thousand people crammed into several acres for the concert. But, concert-goers can't see the fireworks due to trees along the river. So, it's either fireworks or the Boston Pops concert. When my wife and I went in 2008, we found a 1 ft squared piece of land, and we were able to watch the concert. The subway ride out of town was next to impossible. We actually parked near the USS Constitution, paid for parking for several hours, and walked a couple miles to the "Hatshell".
If you don't mind dealing with terrible traffic, tons of people crammed into a relatively small area, there is NOTHING in the country quite like Boston's 4th!
TS