I'm just finishing up my undergraduate degree in computer science at UC Davis, and we used a forum in almost every class, so I saw a lot of them. I can remember four different platforms:
- Smartsite. This is a big system for managing courses online that, as far as I know, the school pays a subscription to use. It has a forum feature which just recently gained math abilities, although I haven't tried them out. Several of the TAs I knew had nightmares about getting the forums set up on Smartsite; one refused to ever use it again, and switched over to Piazza. From a user's perspective the forums weren't that bad, although they were pretty minimal.
- Piazza. This was my favorite of the systems I used. It was the most convenient, in that I could easily reorganize the posts, search among the posts, make new posts, and reply to other posts. Unlike Smartsite, it marks posts as read when you read them; you don't have to click an extra button to mark them as read. Again, I never used the math features. (None of the math classes I took had much of an online presence.)
- Newsgroup, viewed with Mozilla Thunderbird. I hardly ever used it because it was sort of a pain to work with. It felt primitive. I tended to forget it existed because it wasn't in my browser with everything else. I never bothered to configure Thunderbird to work with it properly, so the post titles would cut off after about thirty characters.
- Facebook. One professor here is famous for using a class discussion forum on Facebook. It's not a very good choice. The posts aren't ordered chronologically; they're ordered according to--well, I never did figure out what. (I don't use Facebook. This class was my first experience with it.) You also can't make subgroups or subfora that I could tell, so you can't, e.g., make a "Homework" section with all the posts about homework and a "Tests" section with just posts about tests; you have to shove them all on the main page. It's difficult to find a specific post, too. I once needed to review a post from the beginning of the term, and it took me almost an hour to find it.
Of the four, I liked Piazza the best. It was easy to get started with, which I think is an important feature; some students won't use the forum at all if it's too hard to get started. I know I put off visiting the newsgroup for about three weeks because I wasn't looking forward to setting up Thunderbird to view it, even though that setup was pretty trivial. And Piazza apparently has LaTeX support, so it should be easy to enter math formulas. (Depending on the level of your students, this could be a nice, gentle introduction to LaTeX for them, too.)
I found the forums really helpful because I could ask questions as they arose, instead of having to save them all up for office hours, and it was probably nice for the TAs to be able to answer a common question once, instead of twenty times in office hours. The forum is also an excellent place to form study groups. The professor who used the newsgroup gave us all extra credit for posting a question within the first week, but in general I don't think this is necessary. Just make sure you or one of your TAs is regularly in the forum answering questions. The students will come. We want the help, and this is an easy way to get it.
(More than extra credit, I would have liked to be bribed to post in the forums with recommendations for grad school, but that's just me :)
If you're concerned that it'll end up with students asking all the questions and the TA answering them all, I've seen that happen. We students often don't feel like we have time to help the others by posting to the forum. Personally, I enjoyed answering questions in the forum, but even so, I felt pressured not to spend too much time writing the answer. But even forums with just the TA or the professor answering questions can add a lot to the students' experience, and it's another way to connect with students.