Considering this park is right smack dab in a huge metro area, I almost give it a five star rating. I can't think of any other major metro area that has a park as big and somewhat wild as South Mountain. If you've never been, you definitely need to go! Easy access to a quick hike. Excellent views of Phoenix and surrounding areas. If you're willing to work for it, you can almost find just a little bit of solitude farther back in from one of the less popular trail heads. If you're interested in meeting other folks (and their dogs) who love to hike, you'll meet them! Everyone I've ever talked to on South Mountain trails is friendly and in a good mood. How can you not be in such a place?!
I give South Mountain three stars instead of five because if you DO have time to drive a little ways, there are some awesome places that provide a much better all around hiking experience. From South Mountain trails, you see Phoenix, but you also see how polluted the air in the valley is. While it's less crowded than Piestawa Peak or Camelback Mountain, it's still crowded, especially driving in the main entrance or at the Pima Canyon trail head. And there is less vegetation and other interesting features than on trails farther out.
Overall, South Mountain offers an excellent hiking experience right in the city. It was where I started hiking as a newbie a few years ago. Back then it blew my mind. Now I go there when I don't have time to drive 30 or 60 minutes out somewhere else.