As a 12 year veteran I'll add my .02.
First and foremost, the organizers have managed to keep this massive festival fresh and interesting year after year. An amazing feat unto itself. In addition to seeing the well know it's a great opportunity to be exposed to new artist and music. This year I saw:
Eric Burdon and the Animals, great show, 3 curtain calls. Very rowdy until the house lights came up and everyone magically transformed into grey hair, bifocal wearing geezers like myself.
Sophie Hunger, an unknown Swiss chick who cast a spell over the entire audience. No one but no one wanted it to end. It was at L'Astral which was a perfect venue for this show.
Nikki Yanofsky, local girl does good. She is a 16 year old Montreal chic well on her way to stardom, yes she is that good.
Jeri Brown, Jazz singer from Toronto played the Upstairs Jazz club, perfect venue for her, great show.
Joan Armatrady, probably mis-spelled her name, good show, I like her stuff but caught myself checking my watch. Maybe it was the venue.
Gil Scott Heron, Mr No-Show. Apparently he has a reputation for that, wish I'd know that earlier. Got the run around from Club Soda and Ticketpro about the refund. Jerks.
I came out of the Sophie Hunger concert absolutely spell bound and was surprised to find 150,000 people out in the street jumping up and down to the sounds of a Klezmer band of all things, yes it was the No Smoking Orchestra that chercherfemmes mentioned. These guys were 'right off the boat' but man could they play. Great show. And this was at 11pm on a Monday night.
If you've never been to the Montreal Jazz Festival it is an incredible experience. Most of the shows are free, although there can be really large crowds it's very well run and organized. Most of the music isn't tradition Jazz. There is alot of Blues, Latin Jazz ( high energy stuff ), some rock, and even a couple Brazilian divas throw in. Very lively. The impromptu jam sessions at some of the local clubs are off the charts.