I've never had anything but bad luck with purchased traffic, except for Google and Yahoo. I tried MSN but didn't get enough clicks. Yahoo has been getting better lately, sending me more traffic. You might want to submit to a handfull for quality directories where you might actually get some traffic instead of just PR. Bill
Start by using every free tool you can get to increase your traffic. Also remember that the quality of the content on your site is directly related to the quality of organic traffic you get from search engines. Avoid traffic exchanges like the plaque they offer extremely poor traffic and google will pick up on this immediately. The poor traffic will also probably get you smartpriced.
content is a king more content - higher positions in search engines high positions - more $ imho - traffic from search engines is the best
so far I have seen excellent results with some purchased ad spots on various ad companies like adster and adbrite. But what I still didn't get an answer to when does adsense ban or warn someone if they dont like source of traffic? When traffic increases quickly or when they are about to payout?