I'm of the opinion that any time you go to a dog park, regardless of how well your dog is trained, there's a risk. Dogs are animals, and as such there's always the chance there could be a scuffle or a fight or just a freak accident. It happens.
That said, there's a dog park attached to my apartment complex and I love it. I know most of the people and their dogs, and I know exactly which dogs that my two dogs love to play with or don't particularly get along with. Granted, my Boxer gets along with everyone, so he's not really an issue. If another dog tries to start something with him, he'll just back off.
My Shepherd, on the other hand, isn't opposed to accepting a challenge from another dog, but I'm at the point now that it's easy for me to recognize when things are getting snippy and quickly recall him. He automatically sits and waits when a dog enters the park, and that helps immensely in avoiding problems. I've even got him to a training point where he will actively avoid playing with little dogs and dogs he doesn't particularly care for, thus eliminating problems before they start.
Really, though, the park is just great for burning off excess energy, especially after I've been at work all day. I can take my dogs on a long walk or jog, but nothing tires them out quite as well as thirty minutes of hard running in the park. It's actually kind of nice letting the Shepherd (he's still got that puppy energy) get a nice run in at the park before taking him on a long walk. He behaves so much better after fifteen minutes of running around like a jackass.