I haven't used Powerset much. I have used FAST a lot. I used their API fairly extensively. I could write a book about that (something i have considered - see my "previous life"), but i won't go into all that just now.
What i DID like when i used Powerset was the "factz" (in itself a slightly ironic spelling). You see FAST is all centred around Nouns - people, places, things, authors, books and so on. This works pretty good. (So for example) You search for Steven Livingtone and you get my books, my friends, my bookmarks, my relations and so on - nouns related to me - in facets.
However, what i liked about Powerset what the fact the use verbs in the facts. So again i search for Steven Livingstone (someone has to) and i maybe get "writes", "likes", "bookmarks" and so on - well something to that effect.
Unfortunately i'm not entirely sure how to get these factz appearing (you seem to need to do the search and then select something and even then it isn't always verbs), but i DO like the idea of making a search and combining the doing with the object.
"Steven Livingstone writes MSXML" and so on. One part powerset, one part fast.
I'm still looking into Powerset so things may become more obvious to me.