Piazza is great at what it does: enabling discussion and making have a rolling wiki where they can talk to course staff about issues with the homework. However, they're alienating me with the career-recruitment spam - especially because it's not optional. Plus, isn't this all the more alienating for humanities courses? If it bothers me, and I'm enrolled in/TA'ing cs classes, it must bother the hell out of humanities majors with classes on piazza.
From a student's perspective, Piazza is really nice for communicating. I have never seen any of the professors at my university use it for anything else. They mostly use Sakai, and I don't really have any complaints about it. (and on an unrelated note, they have complained about getting spammed by Piazza)
Not quite. Piazza allows instructors to post resources as well. I've taken a few courses that centered themselves around Piazza and the Piazza course page.