I'm extremely biased (as a co-op member) but the Art is the best place I've ever seen a movie at.
The place is an old-school art-house theater with one screen. The showtimes vary a bit day-to-day, but generally they show more recent independent features during the daytime and second-runs at night. Tickets are really reasonably priced (I think the student price is $7) and concessions are great. I don't like popcorn, but the cookies are $3 (and Pekara made, sweet!) and they offer mixed drinks for $6. I'm especially fond of the mixed drinks, which feature really good spirits (their bourbon is Makers 46, which... wow.)
The movie selection is also great. The independent films are almost always good and very rarely super-pretentious, and the late night films are always good. The first time I saw Cabin In the Woods here was my second favorite movie experience ever; the crowd was raucous and engaged. Plus, they show cult films (do not miss the annual showing of The Room in February- weekend showings are the best) and occasionally have film festivals or special events. Recently, they brought in the Andrew Alden Ensemble to play a live score to Vampyr, which was unbelievably cool.
The Art's fantastic. It blows other art-house theaters I've been to out of the water. Hands down my favorite spot in Champaign.