Given a choice between the path and the roads I would have to ride to commute in, the path wins hands down. It can get crowded during the evening commute, but generally there are more bikes than walkers/runners and more walkers/runners than rollerbladers (the ones I really worry about.) In the morning it is about 90% bikes - I saw only 2 bladers in 11 miles this morning.
I'm riding into Boston. Our roads generally suck and the possibility of getting doored is very, very real.