Took a ride on April 30, late afternoon. Beautiful park, with very nice facilities. This was my first ever ride here, as I am from Virginia and am just visiting the area. I only had time for the Green Loop.
For starters, the park is beautiful (for being in Wisconsin). Good diversity of foilage, including a pretty section through some pines. This is a ride that you should slow down a little and look around at the surrounding environment your first few times through.
As for mountain biking, I found the trail to be somewhat easy. It is not very intense; rather, it is more of an endurance challenge. There are some punishing hills, however there is plenty of time to recover which makes the ride seem easier than it really is. Also, I did not find the ride to be very technical, except in a few spots. There are just too many stretches with very few rocks and roots, and the trail is generally pretty wide.
Despite lacking a technical challenge, this trail is a lot of fun. Riders seem to get the most out of their trail, and I was impressed. Some trails just plain kill you, and others are just plain fun. I would put this one in the fun category. It may be good for some beginners who have good endurance and want to improve their skills on the trail.
My biggest beef, however, with this park is it is way too pricey. You have to pay (a lot of money) to park and then to have a trail pass as well. This is a major drawback, and in my opinion hurts the park's ratings. Furthermore, it is not a very convenient location (although I may be spoiled by the numerous parks near my home in VA). If you are going to this park, I suggest giving yourself a lot of time so you can ride multiple trails.
Overall, I had a lot of fun with this trail and consider it more of a tiring joy ride than anything. Wisconsin should be very proud of this park, despite its high prices and high traffic.