Piazza has been very effective in my class, and I would recommend it highly to my colleagues.
I'm glad to be using Piazza this semester. I'm using Piazza to: 1) push announcements to students and to advisees, while keeping a copy online for reference; 2) receive homeworks and scheduling requests for student talks, in a way that keeps a mutually visible log.
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 Blackboard, users are automatically signed in to Piazza, so there's no need to remember a separate user name and password.