Also, generally speaking college or any school you go to will be what you make of it in the end. I know people who went to party schools like SIU that never left their dorms and were by my standard boring and miserable in college. On the flip side the BEST parties and BEST college weekends I had by a long shot were in Dubuque Iowa partying at schools with undergrads of like 2,000 students. Really it comes down to finding a school that you are comfortable with in the important areas(living arrangements, distance from home, education) and the fun will always be there somewhere.
NIU isn't the perfect school. But I'd rate it from my perspective as above average. Undergrad there is like 18,000 students. If you are looking for a big school experience it's really solid. If you are more into the small school experience, it's likely not for you.