We used to go a lot, but my car was broken into last month while we were there, and I just haven't felt comfortable going back since. But, I can't really recall having issues with dogs. We had an incident when Juno was about 6 months old, but luckily I've never seen the other dog at the park since then.
You just have to constantly be watching the dogs when you're there. A lot of people think they're able to "check out" when they get there, they just let their dogs run wild and don't supervise them. But not only do you have to watch your dog, you have to watch the others as well. Juno can usually find someone of her energy level to play with, but if she doesn't, she can turn into a bully. That's when we leave, or at least walk around a bit so she's not bothering the other dog. (She doesn't get mean, just demanding--in your face bark bark bark).
Always a risk though. Dog daycares that evaluate the dogs they let in are a safer option... too bad they're not free!