I pretty much avoid them.
We have a little one very close to our apartment that is rarely used, so we go to that one from time to time. But there are rarely more than 2-3 other dogs in there at a time (we typically have it all to ourselves), so it's easy to get a feel for the play styles. It's also usually frequented by the same dogs, so we know which ones do well with our dog and which don't. Even there, I always am relatively close to my dog and am ready to remove her/get between her and another dog at the first sign of trouble.
I would never take our girl to a bigger/busier park, honestly. Too many careless owners and poorly behaved dogs. Too risky, in my opinion, though I have a lot of friends that have had great luck with them. Also, with certain breeds (dobes, bully breeds, etc.), you also run the risk of being considered at fault whether it was actually your dog starting the trouble or not.
For people that do like to use them, it's super important to really know your dog and his body language as well as have an excellent recall in place. Dog park dogs should also have a pretty flexible and calm play style. I see so many dogs at dog parks that obviously don't want to be there (nervous or fearful dogs or dogs who just don't enjoy playing with lots of other dogs) or dogs that are downright rude - these types of dogs can cause major problems. Our dog really doesn't enjoy having to deal with rude dogs, so she's usually just as happy to be on a leash exercising with us.