I really like the concept of a shared gestalt setting, as in Lovecraft and his croneys, so I don't have a problem with writing using someone else's creation. In fact, I'd go further in disagreeing with barsoomcore; some of those considered the very best writers in the English language have borrowed more than they invented in terms of plot, characters, and more (ever read much Shakespeare? Tennyson? Mallory?) Same can be said of other languages: Homer didn't invent the characters of Odysseus or Achilles, and likely didn't invent the plots either.
That said, I don't think many of Lovecraft's little group of friends were particularly talented at all; in fact, I don't think Lovecraft was that great of a writer. He had great ideas but his actual writing techniques were often extremely poor, and most of the rest of the Cthulhu Mythos writing circle didn't even have that; they were merely aping Lovecraft himself.
I'm not entirely sure that even that can be said about Greenwood; I find his prose tedious, and I don't think very many of his ideas are even really that good.