No dog parks for us, for three reasons.
1.) I have a very small dog. While he is great with other dogs, I don't trust other dogs, especially larger ones that have never met him before, not to hurt him. A rambunctious Great Dane can squash a Papillon even if they are just trying to be friendly.
2.) I have a dog who, although very friendly, is picked on by other dogs. Border Collies have a different and slightly intimidating way of communicating, and I'm sure that non-herding dogs interpret my BC's behavior as threatening or irritating. I don't want to subject him to being picked on because of that, especially because he is still a puppy and the last thing I (a novice handler) need in a performance/show dog is dog-aggression or fear issues.
3.) Above all else, I avoid dog parks because of the irresponsible owners. People don't hesitate to bring dog-aggressive dogs to the park. Sometimes it is a man with a grotesquely swollen ego, just wanting to show off his "badass" dog by letting it rip into innocent ones. Other times it is a completely ignorant idiot who thinks their dog attacking others is "cute" or "just playing". I can control a pack of dogs by myself with a firm tone and some body language, the dogs really are not the problem. I just never want to deal with the owners, who spoil things for everyone.
If there is an idiot-free dog park somewhere, please send me the address. I'd love to go.
As far as my opinion of them goes, I believe it depends on the park and the dog in question. If a dog is sound and can handle the rough play and threatening behavior that will undoubtedly come up at a dog park (it is a rather unnatural situation for many dogs) then go for it. However, I think that the majority of "friendly" pet dogs should stay out of dog parks until they are well trained.