It's more similar in concept to Pandora - you tell it what content you're interested in, it tries to generate a content feed you might find similarly interesting.
This does not mean it's a poor source for information, quite the contrary - it's a very useful information discovery tool.
However, it will not show you every post (unread or otherwise) from RSS feeds of your choosing. It's just not that kind of tool.
Here's an (older) article on NYTimes regarding it:
http://bits.blogs.nytimes.com/2012/04/03/prismatic-hopes-to-...
This new entry page on their site seems to be an extension of their Google Reader integration they added last June, automating the import of your current RSS feeds as 'seed' interests to generate your Prismatic feed:
http://blog.getprismatic.com/blog/2012/6/1/prismatic-integra...