I've tried Adwords a bit before. I think it's better to buy more targeted ads with BSA for a forum, so you can be certain that those you advertise to will know how to use forums.
I've never used Adwords for forums, but I have advertised online stores, and it turned out a lot better than I expected! I had a nice uptick in sales for a few days, but the best part was that my ranking seemed to improve after a campaign or two. Really weird. 😛
I think google adword for forum is wasting money you know. The meaning of forum is useful information. You can create useful information and then visitors will come
I've used Google Adwords in the past. Both for business sites and forums. It can get VERY expensive. Short answer: Unless you want to spend hundreds per month with Adwords, find a alternative.
I have used Google Adwords before, it generally has helped for me when I needed an extra boost of potential new users on a forum or website I wanted traffic on.