Foundation Park is a really cool course by Illinois standards with some excellent terrain. The elevation change is good for Illinois, but not necessarily amazing by general standards. There are some good water hazards, excellent shot shaping and mature trees, and the course is set in a very well maintained area. It is an experience to be had.
All that said, I feel like this is one of those courses that has been around long enough that its rating is higher due to being evaluated in an era when there weren't as many courses. This is an excellent course, and maybe deserves as high as a 4.25, but in my opinion Canyons blows it out of the water for best in Illinois, and I've played tons of courses more worthy of a 4.5 than this one. It's certainly worth the visit, and is one of the better courses Illinois has to offer. I didn't have time to play the Rec course here but having two courses in one place is always a plus. I also enjoyed the nearby Carlyle City Park nine hole course, but it appears I was in the minority there.
The creek hazards on this course stood out to me, with throws over the windy creek on hole 7 somewhat reminiscent of scenes at Idlewild. There were a few muddy places but overall the course was pretty manageable. Waterproof shoes are recommended if things haven't been particularly dry. Navigation here could have been better. There were next tee indicators on some baskets, but some were missing or facing the wrong direction, and there were no hole numbers on these older baskets without top bands. The course is certainly showing its age, but it has been maintained fairly well.
The maps here are decent, and there are other minor amenities, such as restrooms. This is not a heavily wooded, highly technical course but does require some decent shot placement ability. Most of the shots you're capable of will be required at some point. A lot of fallen trees left in place add to the interest on this course.
This was a pretty typical length course for me at around 90 minutes to play solo, or maybe slightly less. I played the red (short) tees since they were concrete, but in either configuration this is a pretty long course in terms of hole length. Other reviewers noted poison ivy... well, I suck at identifying poison ivy, but I caught poison ivy on this roadtrip for the first time in 18 years of playing disc golf. Did I catch it here? No idea, but be aware. It sucks
For Illinois, this is a fairly pretty area with an above average course. However, it is not in the upper echelon of disc golf available with all of the incredible courses out there now.