Forums are something I have a lot of experience with -- I've been running a very large forum for more than 10 years now. The forum has over 3 million posts and nearly 40,000 members at this point. It's also revenue generating.
You're right that participation is a big factor -- no one really wants to hang out at a board that seems dead, but getting folks to participate can be a bit of a challenge. The way I've done it is to get moderators early into the game, as they stimulate discussion, contribute their own voices, and keep a board a good place to visit. You need to hand pick them, train them in the values you want for your site, and keep in regular touch with them. Keeping the numbers of forums in the board limited is also important -- you need a critical mass in at least one forum to get things kickstarted. Using decent board software is also really helpful, as if there are too many bugs or people find it hard to login/post, they'll just give up. I like vBulletin personally.
A forum, when done well, is a HUGE asset to a site. In my case, my entire community revolves around it. And keep in mind that many of your posts can be indexed in Google, making it easier for people to find you. Forums are also "sticky," giving your readers a reason to come back and back again.
On the flipside, a forum can be a bear to maintain, especially if you get cranky posters. Software updates can be tricky, though this has gotten much easier in recent years. And, of course, a successful board needs a solid backend, so your costs can rise.
I do recommend forums, but I don't think they are for every site. If a good forum exists on your topic already, it's still worth considering. If two good forums already exist, you'll probably find it very hard to get visitors to stay. If you do decide to do it, don't expect folks to rush over right away. If you have an established reader base, you're ahead of the game. If not, you'll want to seed your forum with posts, and answers to your posts, using a variety of usernames -- and invite anyone who drops by your forum to post, whether it's through a request on your site, in your articles, or an e-mail once in a while.
I'd be happy to help with more specifics -- feel free to contact me.