I have been using Piazza ever since you introduced me to it. Every semester I have received very positive feedback from the students about this system. Next semester I will be teaching a graduate seminar course and will be using Piazza again.
Frankly, I can't imagine teaching without Piazza.
I've found Piazza to be rather useful in my upper division undergrad mechanics class. Basically the students have just used it to ask questions about the homework problems. They help each other, and I can help everyone without having to repeat myself. Students of all ilk benefit from this.
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 Blackboard, users are automatically signed in to Piazza, so there's no need to remember a separate user name and password.