I don't think that's likely to be an issue in this case because the ads are not presented in a standard ad format, instead they are part of the usual list of reddit items.
Yeah, probably not good for ad-based sites, but those usually don't have high enough revenue/user to make advertising cost effective anyway.
For a cool new consumer product or service that makes money directly, though, this could be great. When I tried it, the CPC was very low. Whether it's worth it depends on how good a fit the thing you want to advertise is with the audience of Reddit.
I guess it would be a good way to get some interest in a new service, thinking of it similar to a HN review my app post which you can keep at the top for more than a couple of hours.
I can't imagine making a great return selling something directly on there unless you product was so cool that a regular post would have had nearly the same effect.
