To call these stories "collaborations" seems ridiculous to me, since Lovecraft seems to write the vast majority of the story it seems. The so-called "collaborator" seems to throw out a VERY general idea. In my mind it basically goes like this, "say H.P. how about a horror story that takes place on a beach?" or "how about about a love triangle that involves a wizard?" or "a wax museum is a really scary place, I bet it would make for a spine-tingling tale!"
While this is true, there's one thing that really marks the "collaborations" apart from the rest of his work: quality. With maybe one or two exceptions ("Under the Pyramids" and "Curse of Yig," specifically), the team-ups are all pretty shoddily slapped together. You can tell he's slumming for a paycheck.