As a search tool it is more interesting than useful, shining in only a few, pre-selected cases. The advantages over Google are so minimal and the defects so large that I would never consider using this as my main means of searching Wikipedia, let alone the Web at large. To me this product smells like a tech demo, not a fully-featured product launch, intended to convince someone outside Powerset that they really are producing something amazing.
20 bits on Powerset