Was it the end near the clam shell? That's a really tricky junction. I always need to slow right down and check all around a few times before going through there.

As for the bike path vs. Comm Ave... I find Comm Ave and Beacon St through Back Bay too long and flat and boring. The path has lil' animals and a nice view across the rivers and nice trees and such. Riding along there is a nice way to chill out before and after work.