Depending on when you go, Cafe Paradiso's atmosphere changes - but the environment stays the same. Semantics, I know, but I'll put it this way: The cafe opened in August 1998 in the hands of Melissa Fanella. She formerly operated an art gallery in Mount Prospect, so it follows that the cafe doubles as an art space. Since it is located on-campus and close to residence halls and apartments, it attracts a lot of students in the evenings. The cafe is open 24 hours/day during finals week in December and May -- it's fairly crammed at night, so stake out a spot early, and order an espresso (or three) to fortify yourself.
The cafe serves often-homemade deli fare (very yummy sandwiches and soups), baked goods, and coffee and tea-based drinks. In the summer, strawberry italian sodas (extra sweet) are my favorite. The seating is eclectic, and divided into smoking and non-smoking -- but it doesn't matter where you sit, you always come out smelling like smoke. (Not the greatest thing for us non-smokers, but if you go during the day, you can escape the cigarette clouds.)
They also have board games and books; the usual coffeehouse diversions.
This is one of my favorite places to go on-campus. The staff is friendly, and I'd say that they like their jobs (and their boss, who regularly contributes time and use of the cafe to community events). Their tip jar cleverly reads "Support Counter Intelligence." Please do.