I've done this trail system 4 times. Let's preface this review with that, as I don't typically do a trail system more than once.
This is, by far, the best trail system I've ever set foot on. There are several distinct trails, but the two that are my favorite are the Rockafeller Trail (12 mile loop) and the trail to the Falls. No matter what group you take, you will not be disappointed.
The Rockafeller Trail is a challenge. It encircles the entire park, and descends and ascends the mountain. You will interconnect with each trail in the park, so you can easily turn this into 15+ miles, but the main trail isn't that far. The views from the the top of the mountain are amazing, and there are several overlooks that give you a great view of the falls. Use caution: I've seen someone fall off the edge once.
The trail to the bottom of the falls, though, is the best part of this park. The descent/ascent is difficult, but worth it. Once you get to the bottom, the river gives you some great photo moments. Follow the river the waterfall, and just enjoy the view! The adventurous can take the side of the pond and climb right underneath the falls. The sides are slick, so beware. And don't go in the pond. State law.
If you hike one trail system in this state, this is the one. Period.