I really love Piazza. I'm using it for a graduate-level class in machine learning and it's really helping the students to discuss difficult conceptual problems.
One student wrote in anonymously on Piazza to complain a take-home exam interfered with a big event at their fraternity. Several other students wrote in to tell the complaining student to stop whining, that there's plenty of time for the exam if they only budget their time. In the past, I've always had to respond myself. With Piazza the student's peers did it for me.
As a lecturer for a large class at MIT, what I like best about Piazza is that my teaching assistants spend significantly less time answering repetitious questions from students, or correcting simple errors. Now they can spend more time helping me create new material for the class.
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 Stellar, users are automatically signed in to Piazza, so there's no need to remember a separate user name and password.