Have used for a while and it is the only thing I've found that rivals Google Reader. Google Reader is meant for "superconsumers", in the sense that you get the full stream of articles and you decide what to read or not. It is great but demands a lot of time and attention. Prismatic goes around the problem by offering you choices about subjects or categories and searching Twitter for the most shared items in that subject. It "learns" to make better suggestions through your shared and favorite items.
So in that sense it is better than Google Reader, but I cannot say it is a complete replacement. If you need to discover new things outside your usual feeds Prismatic is the ticket