Do not use the meta keywords tag. Many people still think of this as a quick fix for SEO. It's not, It only benefits your competition!! how so? well back then search engines used to rely on Meta Keywords to guess which keywords were relevant to a webpage , now the search engines are sophisticated enought to examine the keywords in the body of a webpage.
by placing Meta Tags on a page your giving your competitor a list of important keywords.can then use these keywords and buy PPC ads or optimize their own site for your important keywords. Why give them this sort of business intelligence? Having invested time and money on exhaustive keyword research to identify the important keyword phrases to use for your own SEO and PPC efforts, why on earth would you make a list of your high value keywords public?

I disagree with your point about meta tags. Google still uses meta tags extensivly for their keywords, and what your keywords are, is not a top-secret item. All your competetors have to do is read your page to figure them out. Plus, optimizing ad's to a site will drive more traffic, and then google adsense will provide a bigger kickback.