They haven't taken an official position in their Webmaster Help section, but general consensus seems to be that MSN does place some value on these tags.

Yahoo

I couldn't find anything "official" but consensus seems to be that Yahoo uses them, especially description tag.

Google

The industry opinion is that Google doesn't bother with meta keywords at all, and that it MIGHT give some credibility to meta description. However, while good meta tags probably won't help in Google, bad ones can hurt. If Google sees words in meta description and keywords that it doesn't find on page, that can have a negative affect.

The Bottom Line

Better to have it and not need it, than need it and not have it!

SEO is not marketing! SEO is building a site so that it's easily found, crawled, indexed and ranked by search engines. You don't see results in a week, or even a month. You can't directly calculate a return on investment.

If you're thinking about SEO, do a baseline analysis of your traffic to determine:

1. The average number of unique visitors per day over last three months

2. The average number of page views per day over last three months

3. Your position in organic results for they key terms you want to target in Google, Yahoo, and MSN

Now do your SEO, wait 6 months, and repeat above. If results are same, or only a little bit above where they were, then your SEO wasn't effective. But if there's a significant difference, and you've got a good deal more traffic than you did before, it probably was.<