The Joomla extension J2XML Importer enables you to batch migrate articles from the Joomla backend. I used it to migrate the contents of a Joomla 1.5 site to a 1.7 site.
First I downloaded the contents from the 1.5 site using J2XML:
http://extensions.joomla.org/extensions/migration-a-conversion/data-import-a-export/12816
Then I used the Importer to import into the 1.7 site:
Details are here: http://extensions.joomla.org/extensions/migration-a-conversion/joomla-migration/15807
You could use it to batch download articles, all in one go if you wanted, but If you have a lot of content, like I did, it makes more sense to download category by category. Use J2XML for a 1.5 site and J2XML Importer for a 1.6/1.7 site. It’s very easy to install and does exactly what it says. And it’s free, though you can donate to the provider, which I did.