Piazza is great... the features that I use work well. The site feels rich and responsive, like a native client app.
Discussion tools need to be nearly perfect (in terms of ease-of-use and functionality) to entice people to actually use them, and Piazza is the closest-to-perfect tool that I've seen for online discussion.
Piazza makes electronic communication to my class absolutely seamless, and it gives the class a wonderful active forum for discussing the material with each other.
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.