Great park! We were introduced to this place yesterday with a Toronto Dog Meetup Group. It was fairly crowded on a Saturday morning, but we still found a parking spot without too much trouble, and everyone we ran into was quite friendly.
The dog park isn't completely fenced in, but it's a giant area surrounded by hills.
My girlfriend and I have been nervous letting our 13-week-old puppy off his leash so far, but felt comfortable letting him roam free here. He got to run all over while meeting dozens of new dogs and people.
The other reviews and people in the park said that this place was almost never empty. We'll definitely be back, it's the perfect place for puppy socializing!