This is just my opinion, but the only reason to use adwords is to drive people directly to a product, sales, or squeeze page. I have dabbled with it for traffic, but for the cost per click compared to bringing people in to browse around....it's not worth it.
Better to promote a blog through normal and free channels and rely on your
SEO, Content, Social Book Marking , and other ways.
I do agree with paying for listing like in Blogger and Podcasters Guide ($5 a month), but I don't think I would pay for Adwords, unless your landing page is a sales page.
JMO