I completely agree. I co-run a hobby based site which is very content rich for a very niche widget market.
It's a busy site for it's market (a few hundred visits a day and a very active forum - it's a very small market).
I'm not sure whether or not it's worth it? We wouldn't bother our users with adverts unless it's worth our while, and with that kind of visit stats I'm not sure it is.
I'd love for this site to pay for it's own hosting by using something like this, that's why the concept is so appealing.
It's difficult to make a commercial judgment over an essentially non-commercial site in the first place, but when you have no idea of pay per click figures, it's almost impossible.