I agree with incredihelp, my site www.dizzidezine.com.au ranks well in various keywords for my industry, but the descriptions change continuously when the bots do a new crawl.I have also found its always one of these:

To quote incredihelp

1. A snippet of text from the page itself.
2. Your DMOZ description if you are in there.
3. Or the part of the description tag.

Generally the description is quite relative, so I'm happy with the changes. In my experience, I try and keep all the info on my site (tags, text, new submissions, etc) relative to how I want the results shown, keep the site informative for the browser (not necessarily the SE's) and most of the time the reults and descriptions are great. I have found the SE's have a good result on average.