Have you ever used a pay-per-click program to promote your forum, such as Google AdWords or Microsoft AdCenter?

These can get expensive, and, since forums usually don't make much money per user, it seems like it wouldn't be profitable.

On the other hand, it can help generate new users and increase the forums popularity.

Do you think PPC is a good way to promote a forum?

I have had good luck using targeted CPM traffic to promote a forum.  The important thing to remember that you have to have the user interact with your creatives in a way that ties into your forum.

With CPM traffic there push, and there is pull.  Push traffic is any sort of traffic where the ad is pushed upon the user (popups/unders).  This is generally pretty cheap, because it's much less effective.

Pull traffic (banners) is generally more effective because the user has already shown interest by clicking.  When you have cool, interactive banners the user is more likely to actually become interested in what they are clicking on, because they are looking for gratification for they time they spent interacting with the banner. 

Myspace is littered with great interactive banners (the little games in banners).  I've had good luck with polls built into banners, that link to poll threads in my forum. 

I agree. Most people only get bot backlinks from ppc. I dont think I would use it unless I was getting a substantial profit from my forum work.