I used it in PHYS 212 and it was extremely helpful as an unofficial course website since our official website is so out of date that simple things like posting files is a pain. The flexibility of Piazza allowed me to create a weekly guide to activities (a "pin"ned note) that not only told the students what the activities were but also how they fit together. And I could post all relevant course materials as attachments on that note. So this was extremely helpful.
In a class of 1000 students it is hard to imagine getting even a few percent of them to come to my office hours each week, to identify where they are having problems and to get the repeated tutored practice that they really need to learn the material. Piazza brings the office hour experience online to the entire class. I can quickly see where students are having the most difficulty based on how many tune in to each of the discussions and on the quality of the questions and student provided answers. I can then easily communicate not only with the student asking a question but with the multitude “listening in” to the answer. With its facilitation of peer instruction, ease of use for communication and powerful feedback for the instructor, I find Piazza to be the most useful tool for improving learning since the introduction of clickers.
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 Angel, users are automatically signed in to Piazza, so there's no need to remember a separate user name and password.