I enjoy using Piazza. Most valuable for me is the fact that it provides a one-size-fits-almost-all solution to student communication. And students can individually opt in to receive posts by email, so no-one can say they missed such and such piece of information.
I use Piazza because its strikes the right balance between flexibility and ease of use. The students like using it, and the smartphone apps are excellent.
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.