Pros
While I've heard that experiences vary from department to department, I was given the responsibilities of a full-time employee at Macmillan--albeit the lowest one on the ladder. I had to do most of the menial work--catalog entries--with a couple of opportunities to draft original content. Got to work hands-on with books that had not yet been published, so that was fun. My boss was a great guy, if a bit unsure of how to interact with me since I mainly took orders from the next highest guy up on the totem pole.
Cons
One's department can feel a bit isolated from the others. Especially as an intern, it can be difficult to sense how you are supposed to prove yourself, move up or contribute in a function outside the generally rote tasks you are given. Partially because the top person in your department might not have a say in how the book gets marketed/produced/etc. So the job felt a little mechanical and impersonal.