Robert_Charlton - 9:49 pm on Feb 23, 2009 (gmt 0)

Is it useful or should I just write a description without thinking about keywords ?

The most useful descriptions are those which anticipate the queries for which the page is likely to rank.

In choosing a snippet that satisfies a query, Google looks for the query terms first in the description, then in onpage text. So, if you want your description to be returned, you do need to think about keywords.

That said, those keywords won't directly help you rank. They do help you with click-throughs.