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I agree with the comments above about forums being top heavy so often. Users may just scroll down without seeing the ads at the top. Of course, any ads in forum pages are going to have very low CTR.
I've had pretty good results in terms of ad targeting - however, clickthrough rates are low, for reasons mentioned above. I'm not worried though, as it's a hobby site, and even the minimal number of clicks I get will pay for the hosting and some new toys.
I've been quite encouraged by users reactions to the ads - I asked what they thought, and reponses ranged from 'I just ignore them' to 'I like them, they're really useful'. This has me considering moving them to the top, but also giving regulars the option to turn off ads.
One concern I do have is that sometimes users comment on ads in their posts - 'Hey look, we're already getting ads about XXXXXX on here, isn't that clever.' was one. Stuff like that might increase clicks and result in an email from the big G. However, I suspect that my figures are so low it might just get passed off as random fluctuation.