Reddit/Digg/HN were never about aggregation for me. I use RSS for that. I come to news sharing sites to talk about the news. Prismatic doesn't look like it enables that.
It delivers a personalized curated news feed which is informed by complex data analysis, recommendation, semantic-filtering and machine learning algorithms that use your elected interests and social graph as a data source.
It's much more than simple aggregation.
You can probably tell that I really like it; and no sir, I don't work for them (!)
The problem is, at least from my short meander over there, was that the categories of things is based on your usage of Facebook, Twitter, and/or Google+. Unfortunately I don't really use any of those things for sharing (only a Facebook account that I never post on) so this site isn't very useful for me.