Well, the site has finally gone live this week. So far, it indexes only a small set of pages – those found in Wikipedia and Freebase. They have plans to index more data sets in the near future.
The consensus seems to be that the technology is pretty interesting, but is not that useful so far. The Times suggests the major problem is that for most people, keyword searches are working just fine. Rumours are already swirling that Powerset will be bought by one of the big search companies who may be in a better position to bring the technology to a broader user base.