Piazza is definitely a big improvement on the university-provided discussion groups we're accustomed to using because we can faithfully represent the instructional roles and organizational system that we use in teaching. All the other advantages of Piazza are gravy!
Piazza easily allows us to pool student questions from all sections of a multi-section course in a single forum where all faculty and TAs for the course can contribute answers. This semester, that has given us an average response time to student questions of about 40 minutes.
I love using Piazza for answering student questions; the user interface is unbelievably nicer than anything else I've seen. It is easy to enter text and easy to see what questions need answering; It is just easy to use.
Instructors can create a Piazza for their class with a single click.
Students and instructors are automatically enrolled with appropriate roles, based on the course roster.
When clicking on Piazza in Carmen, users are automatically signed in to Piazza, so there's no need to remember a separate user name and password.