I think the answer to whether they are good or not depends on the circumstances. On whole, though, I think that they are truly wonderful, subject to a few caveats. It is true that many dog owners have no clue what they are doing, and are not responsible. Often that leads to ill behaved and uncontrolled dogs running around off leash at the park. There is no doubt that when you go to the park, you and your dog are taking a risk that someone else's dog (or someone else, for that matter) won't attack. But, life is full of risk-- when you take a walk with your dog, you take the risk someone won't hit you with their car, or try to mug you. You can't simply hide inside all of your life, so you must simply accept some level of risk. The question is whether the benefits outweigh the risk. I agree that all of the problems that Leeburg warns of are real concerns. But in my case, for my dog at my park, with the people that generally go there, the rewards greatly outweigh the risks.
My local dog park is a city park with, as far as I have noticed, no official oversight. There is also no separate area for big and small dogs, adult dogs and puppies, or any other separation. There supposedly is a minimum age requirement for puppies and people, and a requirement that dogs be vaccinated, though other people often let people in without the requisite pass. So, sometimes young puppies (and people), and dogs with no proof of vaccination get in. For the last two plus years, I have taken my fixed male Berner to the park quite often-- usually multiple times a week, and some weeks, every single day. He loves it, and just absolutely gets excited with anticipation when I come home in hopes we are going. In my experience, most of the people there generally have a pretty good idea of what is acceptable behavior, and what is not, and generally they make a least a passing effort to monitor their dogs. Those people who don't are generally chastised by the others. So, there is some peer pressure to help police the park, even though there is no official oversight. Thus, at least at my park, the people are pretty good, and the dogs, with a few exception, are fairly well behaved.
You have to know your dog, and understand dog behavior and interaction. Many dogs simply do not like being at the park. If that is the case, then don't force your dog to go. Also, I am amazed at my park how many people bring small dogs or puppies in. There is no way I would bring a toy dog or a puppy to a non-segregated park, but I see it all the time. There is just too much chance for injury, even from just over exuberant play. In addition, puppies have a period of development when they are very impressionable to experiences of fear. You do NOT want to take your puppy to the park during this period of development or you can scar them for life. If your puppy gets scared, it might let out a yelp. That yelp can be like blood in the water, and suddenly that puppy is surrounded by a pack of big barky unfamiliar dogs acting frenetically. That’s bad. Then, when you go to pick that puppy up to protect him, you are reinforcing that dog’s fear reaction. That’s also bad.
Assuming your dog is big enough to take care of himself, then you absolutely must closely monitor your dog, both to make sure your dog behaves, and to protect him if necessary, as well as pick up the poop. (I can’t tell you how many times I have stepped in poop.) If you spend some time at the park, and you pay attention, you notice patterns, and you can distinguish between harmless play, and inappropriate behavior. Often other people disagree with what you think is appropriate. Some are afraid of the least bit of play growling, wrestling and chasing (this is okay, as long as they both are giving off happy body language, and taking turns chasing and being chased) others think mounting and aggressive nipping and lunging is just play (its not, and should be stopped). Often people don't recognize behavior that leads to spats between dogs. For example, one dog trying to mount another dog can lead to a fight between two alpha dogs. Two dogs trying to get the same ball can lead to fights. Usually, these spats are nothing more serious than that-- spats. There will be growling, barking, lunging, nipping, and even a bite without serious injury. Usually, these are broken up with nothing more than a few hurt feelings. Of course, on rare occasions, there will be a more serious fight. People really should step in before things escalate. Often people do, but sometimes they don't. My dog is very good natured. He does not dominate other dogs, and is not really afraid of other dogs, and does not overreact to aggressive behavior. Generally, I try to let him handle most situations, and only step in when it is apparent intervention is necessary. On the three or four times I have seen him try to mount another dog, I immediately stop him. Primarily, I do this because I don't want him to engage in alpha behavior because I don’t want to live with an alpha male. But also, if he were to do that to the wrong dog, he could cause another dog to become aggressive. If another dog tries to mount him, I generally let him deal with it, unless the other dog is extremely persistent.
People do bring un-spayed and un-neutered animals. Obviously, do NOT bring an intact female to the park when she is in heat. Unfortunately, the intact males can sometimes tell if she is in heat before you can.
If you pay attention to all of the above, if you make sure you are in a good park environment, and you pay attention, the experience can be wonderful. It is really a great way for your dog to meet people and other dogs. That generally is very beneficial to your dog’s personality. Your dog will get much more exercise running around off leash than just walking a few blocks on leash. But, if you have an aggressive dog, or an overly fearful dog, if your dog is too small, or too young or too old, its probably not a good idea for your situation.
