From the about page: "Our unique innovations in search are rooted in breakthrough technologies that take advantage of the structure and nuances of natural language. Using these advanced techniques, Powerset is building a large-scale search engine that breaks the confines of keyword search." For now, Powerset only indexes pages from the English Wikipedia and it's not publicly available (but you can request an invite).
For example, Google shows almost the same search results for [Pyra Labs acquired by Google] and for [When was Pyra Labs acquired by Google?]. Even if the answer can be found in one of the first snippets, Google doesn't highlight it or display it prominently, like you can see in Powerset.
Do you know a query that shows irrelevant results at Google? Post it in the comments and I'll upload a screenshot of Powerset's results. Obviously, to compare Powerset's results with Google results, you need to restrict Google to en.wikipedia.org, the only site currently indexed by Powerset.
Updated.
Query #1 (from Matt Cutts): How many states are in the United States?
Conclusion: the first 10 Powerset results are terrible. On the other hand, Google shows the answer in a OneBox, but also a strange book search OneBox at the top: "How Many Doctors Do We Need?" by Duncan Yaggy, Patricia Hodgson.
Query #2: What Nobel Prize winners were born in Russia?
Conclusion: Google's results are better even if you restrict them to Wikipedia. The top result from Wikipedia (the third Google result) is a page titled Noble laureates by country. Only few of the people mentioned in Powerset's results are Russian who won the Nobel prize and there's no complete list.
Query #3: Who was the last president of United States?
Conclusion: The results #3, #4, #6 mention George W. Bush, but there are other names of former presidents. Google's fourth result has this title: "George W Bush: Last President Of The United States Of America?".
Query #4: Who are the founders of Yahoo?
Conclusion: the second result includes the answer, but it's only partially highlighted in the snippet. Google's top results has a good snippet: "The two founders of Yahoo!, David Filo and Jerry Yang, Ph.D. candidates in Electrical Engineering at Stanford University, started their guide in a campus."
