My favorite feature in Piazza is the poll. I use it like a clicker to get responses to questions during the lecture. I also like the statistics. I give participation points based on the number of times a student posts. Piazza makes that very easy.
I like the app for my iPhone.
I used Piazza in a large calculus class at UC Berkeley and both my students and I found it extremely useful and easy to use. I plan to keep using Piazza in all my future classes, large and small.
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.