I think this is the best dog park in the Riverdale/Leslieville area. Winthrow tends to be consistently busier (i.e., more of a guarantee that there will be dogs to play with), but it's not fenced in so it can be kind of anarchaic over there.
The ground is covered with wood chips so the dogs don't get too dirty and it's pretty well shaded in the summer when it's hot (although a big tree recently had to be taken down). The best part about it is that the owners that go there are all serious dog people, but are not too uptight. Sometimes at other places if a dog is playing an owner loses his/her mind thinking it's a fight. That's not the case with the owners here.
If you're going on the weekend try to make it before noon if you want to make sure there will be lots of dogs.