This is hands down one of the best festivals in MTL! I am yet to miss a Jazz Fest since moving here, and I don't think I am about to either. There is a great line-up of both FREE and paid shows every year. I'll give you one guess as to which ones I attend. I recommend having an idea of when and where the performers you're interested in are playing prior to going. If you have none go anyways walk around until the someone's beats lure you in.
I was a bit disappointed this year by the random barriers disrupting the walking paths. I'm not sure what that was about. They just got in the way and unnecessarily crowded areas.
That said this place can get really crowded, and I don't recommend bringing a stroller here as it'll just get in the way. If you do bring children along check out the kids area.