Not as important as it used to be. 
Many SEO experts says they are not used anymore by search engines to rank your site, and despite this some of them still use it.
As I was reading on Red Fly Marketing once, and I quote:

Well, over the years as meta tag keyword stuffing became prominent, search engines adjusted their algorithms to give less weight to the tags. A LOT less weight. Now, meta tag weight is so negligible that many webmasters no longer even use them.
You will notice that I use them here. The main reason is because Google will normally list a sites meta description ( Of course we all know this is query dependent ) as the description in SERPS. I would prefer to have control over this rather than let Google choose what to display.

he’s perfectly right.
For the most part, the importance of a meta description or keywords lies in the fact that search engines may choose to use it in the SERP’s, instead of displaying relevant bits from the page content.
A meta description might have a minor effect on search engines rankings, but is definitely not a critical factor.