Unfortunately research shows that keywords aren't really utilized anymore, but the descriptions are important. So since it doesn't generate descriptions for all my old articles it wasn't as useful as i hoped, but for all new content it's good. It's too bad about the keywords, if I would have known this I never would have purchased the plugin...
the descriptions for the old articles are also generated - you can see this in the source-text of the articles - but the description for the old articles is not saved to the database because this is not necessary ;o)
the search-engines like google read keywords and descriptions from the source-text and not from the database. some joomla-extensions use the keywords from the database, thats why they are saved but the description is not needed in the database
seo-generator always generates keywords and description in temporary way and put these into the source-text