I spent 3 years as an undergrad in IST, so my view might be a bit different from that of a 'new' grad student. But the story I heard as an undergrad was that the (undergraduate) program was meant to address an unfilled need in industry for people who could understand more than just technology, for people that could understand the importance of people and information in the technology mix. As an undergrad, it seemed like this meant that most people were destined for the corporate world. And it seems that most IST undergards did very well if that was there goal. But I was focused on this whole other part of IST that seemed a bit less practical, a bit more focused on social or philosophical issues realated to technology. The thread was there so I followed it, but I often felt like I was a bit out of step with my seemingly more pragmatic peers.
IST is organizanized around centers rather than departments, and that is a large part of its appeal for me. If I had to pick a department, I don't think I would be nearly as interested in IST. And my observation has been that there us a great deal of interaction across centers, where faculty from seperate disciplines are working together on research that niether would do as well alone. Whether there could or should be more intereaction across centers and faculty is another question, but it seems common enough so far in my experience.
Right now, I can see myself somewhere between the Center for HCI and the Center for the Information Society. And I ultimately don't see them as seperate things, there is a lot of overlap there and, ideally, I would pull elelents from both of those Centers together to be the kind of reasearcher that I want to be. If they were distinct departments, I think that I would feel a little differently. The message I have gotten from several faculty is that we (as grad students) should be understanding the 'big picture' of IST better than they do, each coming form their own academic background, that we (as grad students) are to be a synthesis of some sort that embodies what IST is. Perhaps this is a bit idealisrtic. But IST stands out to me as a place you could explore almost anything related to techology and find faculty to guide your interest, while also getting a bigger picture than just that particular interest.