My TAs forced me to "get with the program" and use Piazza as our mechanism for student questions. In the past I've simply used a central e-mail address that got forwarded to all the TAs (and me); questions sent there would be answered by a TA (or me) and we all would see the reply.
Piazza offers some additional bulletin-board+ functionality on top of that framework -- things like being able to type things in Latex, easily see what questions are left open, etc. It seems useful enough; it's definitely better than past bulletin-board-style offerings I've seen. Honestly, though, for me the primary benefit is that it lessens the amount entering my mail spool -- it's more organized having these questions all in one place, with formatting. I'll have to see what the students think during and at the end of the semester.
At the moment I'd recommend it, which is not something I thought I'd say -- like I said, I've eschewed such tools in the past. On the other hand, I'm not clear on their eventual business model -- I don't think I'd pay for using it as a service. Perhaps the plan is just to get us so used to it that the idea of not using it is painful enough that we decide to pay. We'll see if I reach that point over the semester.