We stayed in the Turner lake south loop, which has nice and private sites. This is in contrast to Honeysuckle hollow which is open and row after row of RV sites. Fox den and Prairie view loops are nice as well with mostly primitive sites. Be aware that the map on www.ExloreMoreIL,com  (where you make your reservations) doesn't show where the restrooms are. In the Turner loop, the pit toilets are next to the path to the fishing pier, while the showers and full restrooms are between Turner and Fox den, there is a short path between sites 60 & 61. Facilities are clean and well lit.
Fire pits are slightly raised steel troughs with a heavy adjustable grill.
This is a BIG park, the campgrounds are two miles from the front gate, and it's a drive to the main boat launch at Maple Grove, where you have access to the Fox River and Grass lake. This location has a huge parking lot for boat trailers, a very well stocked general store with fishing supplies, a cafe, boat rentals and good firewood.
To get away from the crowd if you're a canoeist or kayaker it's nicer to leave the park and go back in at Oak Point picnic area and boat launch at the northern border. This will put you on the Fox river with developments to the north (including a bar & grill) or to the south which is all nature. I think the distance by boat between the two launches is 4-5 miles, but I'm not sure, Halfway there you'll meet up with the Goldfinch trail which is the shortest loop and the most diverse in term of flora.
There are horse/ snowmobile trails, but no equestrian campground.
Chain o' Lakes is one of the busiest waterways in Illinois, with motorboats everywhere in the height of the summer season. We canoe, and both times we stayed there it was October, so I can't tell you how crowded, but the vast number of picnic areas must be there for a reason.
An odd thing is that you can hear a Steam Train whistle quite often, this is from Richardson's Adventure Farm in Spring Grove. We don't have kids so we went to Stade's Farm & Orchard (which is also large and kid friendly) because they have an orchard and vegetable market.
One last thing, the gates close at 10:PM and there are tire shredders, you can leave but you can't get back in... unless you want to walk 2+ miles in the dark :)