Write a description that would both inform and interest users if they saw your description meta tag as a snippet in a search result. While there's no minimal or maximal length for the text in a description meta tag, we recommend making sure that it's long enough to be fully shown in Search (note that users may see different sized snippets depending on how and where they search), and contains all the relevant information users would need to determine whether the page will be useful and relevant to them.
QUOTE: “Search engine optimization is often about making small modifications to parts of your website. When viewed individually, these changes might seem like incremental improvements, but when combined with other optimizations, they could have a noticeable impact on your site’s user experience and performance in organic search results.” Google Starter Guide, 2008
Sometimes I think if your titles are spammy, your keywords are spammy, and your meta description is spammy, Google might stop right there – even they probably will want to save bandwidth at some time. Putting a keyword in the description won’t take a crap site to number 1 or raise you 50 spots in a competitive niche – so why optimise for a search engine when you can optimise for a human? – I think that is much more valuable, especially if you are in the mix already – that is – on page one for your keyword.
Ever heard of Maslow's hierarchy of needs? It's a theory of psychology that prioritizes the most fundamental human needs (like air, water, and physical safety) over more advanced needs (like esteem and social belonging). The theory is that you can't achieve the needs at the top without ensuring the more fundamental needs are met first. Love doesn't matter if you don't have food.