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.
Piazza does what it should do; good looks; easy to navigate and use. Nothing really that I do not like.
I've been very happy with Piazza so far. We use it for a computer architecture course with 150 students. Last year we used Google Groups, and it was terribly difficult to search for messages, weed out duplicates, and identify good answers. Piazza has done very well on all of these fronts.
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.