I just spent a week there 2 months ago. I hadn't been there in about 20+ years. We went during the week Tues-Sat. The campground was largely unoccupied and quiet during the week, but Friday night it filled up completely within a few hours. I'm glad we left when we did because we went from practically having the entire place to ourselves with peace and quiet, to college kids blaring vulgar rap music, mopeds buzzing by every two minutes, Kids yelling and playing way past midnight, and just general commotion that comes with large crowds. I try not to complain about that stuff too much. Idiots are gonna be idiots anywhere you go... whaddya gonna do? It didn't bother me that much since we spent most of our days in the woods. It's a popular place, so I'd suggest going during the week when the kids are in school.
About the park itself, It's definitely worth the trip if you're a hiker. There are plenty of places to explore on foot. The falls were a bit disappointing for me. The trails leading to them have been mostly blocked off by DNR for safety reasons. My guess is someone got hurt and ruined it for us all. You can still see them thought, just can't actually walk into them and take a bath like before. The falls seemed a bit dried up too and that's just because of the time of year I went. Early spring/summer time is the best when the water is flowing heavy.
There are lots of hilly roadways to bike, and there are plenty of places to hike. Bring a flashlight too... There is a trail with about a mile-long cave that you can hike completely through. Sort of a mini-Mammoth Cave. That is absolutely awesome! Trail raitings I found were inaccurate. "Moderate" actually means you'll bust your butt, and "rugged" means I hope you like pain. This coming from a guy who hikes the Smoky Mountains every year without fail. There were a lot of fallen trees, boulders, and wash-outs on just about every trail. Not sure if it's normally this way, or a maintenance issue, but if you're fit enough it shouldn't be a problem... Just makes things more interesting. Park provisions are limited. There is no on-site camp store or anything like that. There is only a laundry and firewood at the campground gate. Of course, there is your typical nature station. There is also a large swimming pool area which closes in August I think.
The town of Madison is close by and has just about anything you could need in a pinch. There's a walmart, Tractor Supply, Dollar General, and a multitude of restaurants. So you're not entirely removed from civilization. All in all, it's a lovely park and I'd definitely go back!
I'd post a few pics, but the files are all too large. It won't let me post them. Sorry!