Yaro asks Do Meta Tag Keywords Matter Anymore? and does a good job of explaining what they are and answering the question. His advice to whether we should use meta tags:
‘Yes and no. It definitely should not be prioritized and if you have other, better SEO things to do worry about don’t spend time on your meta keywords. If you insist on using meta keywords use them sparingly, only a handful, about 10 maximum, and keep them very relevant to the page content. Less is more in this case.’
I agree with this advice – if I were setting up a new blog I’d include them but it wouldn’t be the first thing that I look at.
‘Meta keywords are a legacy of web 1.0 and are slowly being phased out completely. Your title tags, heading tags and content play a much more important role and really if you have time to spare to work on SEO you should be writing great new content that people will link to, not cramming your pages with redundant keywords.’
Good advice again – however I’d add one thing.
If you’re running Adsense ads on your site then you’ll probably want to include meta tags. Many experienced Adsense publishers highly recommend them – not to optimize your site in Search Engines – but because they seem to have impact upon what ads are being served.