The Google content network is fraught with problems in part because of many weak or even scammy sites set up to exploit the market. While Google has taken measures to clean up the worst abuses, the fact remains that response rates and 'value' on the content network is generally much less effective than on search, largely because of course on search people are generally actively looking for information, while this isn't always the case on the content system.
The content network can work for certain niches and markets, but you need to know your stuff and be sure to bid low enough that you aren't overpaying.
One option of course is to look at relevant and interesting websites which are of interest to your clients. Even if they don't sell advertising directly (they often do), you can specify certain sites on the content network and focus your marketing accordingly.
But frankly I think the time you spend on this sort of stuff could be much better used in developing your site so that you achieve organic search traffic and results. It is often a much slower process (with adwords, you pay and you get your 'ranking' right away) but the results are much more valuable and enduring for your business.
