Word from the Google camp is that the keywords meta tag is officially dead when it comes to optimising your website. But I would still consider using it for other search engines such as Bing and Yahoo.
Google states that the keyword meta tag has been spammed by hundreds of webmasters (usually amateur marketeers) stuffing it with non relevant keywords in hope that they would gain high ranking results. And they probably did for a short amount of time... before they got sand boxed.
It's so important to remember that quality content with right keyword density can be the most effective route to SEO success. But don't expect instant results! In my experience its taking longer to get the desired results straight away, but be persistent if you believe your content is good as it will come!
