It's a pretty big honor at Pitt... out of 14K or so of undergrads, i think it averages about 30 a semester get in. I was initiated fall semester junior year, the first time I was eligible to apply.
They don't do anything much at my school. The only people who ever do anything are the officers, and the main thing they do is cosponsor the Homecoming King and Queen election.
It's fabulous on your resume, though!
