Piazza is a great tool for instructors. I love the convenience of having one place the students can go to post questions to others, look up lectures, hws and vent(!). I really thought it was much nicer than having a web site for a course. I will definitely use Piazza in future courses including graduate courses.
My experience with Piazza has been wonderful. As a grad student I have had the chance to use Piazza for some of the classes that I took (as a student) and for some of the classes that I was a teaching instructor for and I am really happy with Piazza.
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 Desire2Learn, users are automatically signed in to Piazza, so there's no need to remember a separate user name and password.