I've had the impression that google adwords and pay-per-click related programs are useful when one is selling a product or service online. Is it useful simply to get traffic or is it cost-inefficient?
Depends what you're 'getting traffic' for. If you are merely looking to get traffic for AdSense the most likely not although it is possible.
true. Do you mean to populate a forum? Or to to profit bringing traffic to your adsense site from AdWords? Even when you sell products or services is hard to setup and profit from adwords due to the concurrence of the ad space - if you are not selling products/services it is not a good idea to spend money with AdWords.
I'm not concerned with getting adsense money from traffic at this point. Getting many people to my site and getting people to sign up and post on forums is what I'd prefer. By cost-inefficient I meant that for every dollar you spend on adwords you might get few people.
Well, how cost efficient it is will be based on how good you are finding clever ways to target people interested in your site. If you can find ways to do this that not a lot of other people are using then the price may be cheap, though I find that cheap often equates to a low volume of potential visitors. I'm trying some new ideas for one of my own sites right now actually... You could try both keyword and site targeted campaigns with budget limits. If you are able to track conversions you'd easily be able to see how much it costs you per user action (presumably a user signup). Then, when you see something working, this may give you more ideas for tweaking your advertising strategies to bring down your costs per conversion.