Well... I love going to the dog park, and so far, ALL my dogs have enjoyed it, too.
I use it at a teaching tool.
I made sure Denna was mature enough to handle the attention before we went. She was probably closer to 10 months old for our first visit.
Our off-leash park is 40 ACRES... not completely fenced in. LOADS of space, so if you don't like a certain dog, it's easy to walk away (which is what I want to teach my dog... WALK AWAY from conflict, don't "finish it").
Our first visit, Denna was "attacked" by an over-amorous lab who just wanted to hump her.... she couldn't back away fast enough. I went in and rescued her - basically, stopped her from backing up and shoo'd the lab away. He did go away and rejoin his owner (and her other dogs)... and we followed after them along the path - where Denna and the lab re-connected in a more polite way and had a fun game of chase. SO, first "negative" encounter ended on a positive note! Teaching accomplished!
I do stay extra-vigilant when there are off-leash dogs around (at ANY location). Denna is pretty good at listening to other dog's signals, but I don't want other dogs to ignore her signals... and I want to be ready to step in if needed, and move us away from trouble - hopefully before anything bad happens.
Denna was VERY social until about 1.5/2 years old, then she got less interested in chasing other dogs (or being chased), and more into just running around, swimming, and sniffing butts as they passed by. She still has a great time when we do go. I love letting her have space for some MAJOR, unending zoomies - she can really move! (at long as it's in a straight line)
We only go about 6 times a year, and always during the slow time... but the group of dogs and people at the big park (Marymoor, for people in the area) is always attentive and responsible, or so I've found.
Denna also boards at a place with daycare during the day... so keeping her attuned and polite around strange dogs is important to me. Our park excursions help with that effort. But - it's the right park, at the right time, with the right people & dogs there... which is always subject to change.
As for becoming DA without warning... I think you'll have plenty of warnings, as long as you're looking for them (body language, excitement levels, etc.). And as long as you acknowledge them and take appropriate action, the DA should never become unmanageable - although excursions to the dog park may be off the list at that point.