Piazza is an extremely effective and efficient means to help students with tough course material. I have been using it only for this semester, but I am certain to use it in all of my future classes. Features that I believe are invaluable: (1) Ability to write equations with pure TeX constructs. I teach thermodynamics where everything is in complex equations and no way can I deal with equation editors :-) (2) E-mail notification of questions that have been posted, both to the instructor and students. I get e-mails immediately and know that there is a question to be answered. (3) Follow-up capability is great. (4) Ability for students to respond. Survey of students has been very positive as well. Overall very nice product and I've been most pleased with it.
I have very much appreciated Piazza this semester in my parallel scientific computing class. This is a smaller class (41 students in the final tally), but even so, the students have often been good about answering each other's questions before I get to them. I plan to use the system again next semester when I teach the introductory scientific computing class.
Piazza has really changed the way discussion works in the class; there are *many* more questions being asked which can only be a good thing. Piazza makes it easy for us to answer them promptly while only answering once. I love the "someone else is editing this answer right now" feature, which has saved me hours of time I would have wasted answering questions at the same time as my co-instructor.
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.