This is a great park with lots of thrill rides. It's never the same park twice, they are always adding rides and taking down the really old ones. And it's such a big park, if you just go for a day, there's no way you'll ride them all. It's also great for families as they have a childrens park and plenty of children friendly rides.
The only drawback is the long lines. It's a popular theme park for the mid-west and there are always long lines. You'll spend more time in line than on rides. But I will give them credit, they try to put tents over the lines so you aren't waiting in the sun, and sometimes they are selling drinks right there. And honestly, they do keep it moving. There's no fudging when it's your turn. Get in and get going.
You won't regret it, but if you have the time and money, I'd go for a few days and not just the one.