I have been very pleased with Piazza, and I even encouraged my colleagues to adopt it. In addition, I've had multiple students give unsolicited feedback about how useful they find Piazza to be.
What sets Piazza apart from other forum systems is that it has been optimized for classroom use, with so much attention to detail. I can easily tell which threads need attention. I am notified whenever posts are made, so I know when I need to look. If I want to track student participation, that is easy to do. The layout encourages students to participate.
I use Piazza for all of my undergraduate courses. It's very easy to setup. Scholar offers a similar feature (chat), but I don't think it allows a student to post a question anonymously. I want the flow of questions to be completely uninhibited, which anonymous posting fosters. I'm enjoying using it for the first time in my chemistry class, and I will use it for my classes next term.
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 Scholar, users are automatically signed in to Piazza, so there's no need to remember a separate user name and password.