As with most SEO queries there is no straight forward answer, so I’ll just go with it depends…

Basically Google doesn’t use them any more at all!

So if you’re only worried about how you show up on Google don’t bother (unless your site search feature uses them!).

However, if you’re concerned about showing up on Yahoo or Bing they are a necessity!

Bing and Yahoo have a combined 10%* of search market share which may not seem like much first glance, however this accounts for 4.6 Billion^ searches per month in the US alone!

Yahoo has claimed in the past that they stopped indexing the meta keyword tag, however they provide advice on improving your search position including:

Use a “keyword” metatag to list keywords for each page of your website. Use distinct lists of keywords that relate to each specific page on the site. Do not use one broad set of keywords for the entire site.

Bing also provides similar suggestions in their head tag optimisation blog.

So if you care about your position on Bing or Yahoo follow their advice, but like any SEO method you will need more than just a good meta keyword tag to position well on any search engine.

*According to Net Market Share

^According to comScore December 2010 U.S. Search Engine Rankings