Many people who use Google AdWords will opt out of the content network because they feel that since people are not actively searching when they find ads in the content network, they conversion rates will be lower. But in a recent WebmasterWorld thread, Roger Montti explains that the content network isn't as bad as it's hyped up to be. The important thing is that it's really different from the search network.
Roger points out that there are different types of ads in the content network. Some blend in and can resemble navigation. Even contextual ads are different, he notes. But it's similar to the search network too, because some pages have more quality conversions than others:
Some sites convert better than others, sometimes sites you never thought would convert. That's why I am bidding on a wide assortment of sites, then removing those that refuse to convert regardless of the different approaches. In this regard, it's similar to the search network
Better yet, cost per conversion is lower than on the search network. This is probably because advertisers who currently use the content network aren't really acknowledging the differences between the content network and the search network as Roger has done.
The issue is really that a lot of advertisers are a lot lazier -- some may "set it and forget" -- and this is a tactic that won't yield the conversions they're looking for.
Forum discussion continues at WebmasterWorld.