Being a student here for four years, the Illini Union has a special place in my heart. Not only is it the center of student body, but it also serve as a great and convenient study location.
Need a quiet space? Come study in the president's lounge. Need to have a meeting set up with your other student organization members? Go up to the second floor in the RSO office. Don't have time to go back home and make lunch? The basement's got a variety of quick bites at your service. Need to checkout some tech goodies that are vital to your studies/work? Hit up the tech center and they sell a lot of gadgets ranging from laptops to iphones.
A lot of student events are also hosted here in this building, in addition to some dance classes on the third floor. Oh, did I mention that this place is also a hotel? That's right! It is also a great hotel if you are from out of town and want to visit the campus, since it is literally the heart of campus, physically right next to Green Street, which is the lifeblood of the school in terms of restaurants. Definitely check it out!