I personally have used the dog park to socialise my Sibe, who came to me with no socialisation skills, having not seen any dogs from the time he went to his abusive former owners till when he was picked up after being on the streets for 8 weeks and put in the pound (about 2.5 years later). I've found that the key to socialising (and to teaching off leash behaviour) is to start the dog slowly and make sure that you're always able to be in control of the situation. With Zero I introduced him to the park by having him on the opposite side of the fence where he was able to sniff and be sniffed but not fully interact. During this time I made him sit and stay and he had to keep paying attention to me. For a few weeks we did this until another owner told me to bring Zero in and introduced us to his BC who was bouncy but very obedient. By spinning Zero around and letting Marble sniff him first, we put Zero in the submissive position and made his first interaction with other dogs a positive one. Now Zero is fine in the park, though I still follow him around and make sure that he recieves a correction everytime he stands in a dominant position, and praise when he plays gently with a smaller dog, or puppies.
A word of warning though - in the time we've been going to the park i've seen some awful things including a fight betweek a staffy and a great dane which resulted in a toy poodle being killed when it got in the way and caught in the mouth of the staffy. Fortunately Zero stayed well away from this fight but you just never know - there are a lot of unbalanced dogs at the park and some owners who don't bother to watch what their dog is doing... If you have foxtel, there is an episode of The Dog Whisperer with Cesar Milan where he talks about off-leash parks. I suggest trying to track down a copy of the ep - this whole series has helped me more than I can ever tell you!