Clean, quiet, lots of computers, and apparently some guy who yells at you if you fall asleep in one of the chairs.
Lied Library is a great place to study or hang out. There's always a computer open and they even have laptops available for checkout. The quiet floors are where I'm usually at. However, they are not the "absolutely still" quiet that I've experienced in other academic libraries. For some reason, I always see groups going up to the quiet floors to study and they make noise!
As for the guy who yells at you, I've never encountered him but I've heard stories..