Unfortunately, if you've ever read anything about Lovecraft's life, this documentary will probably end up leaving you wanting something a bit more in depth. For one thing, the Wikipedia entry on Lovecraft is actually quite good. It covers everything that this documentary covers, and you get the benefit of seeing citations. The interviews aren't really all that great, but you do get to hear some interesting people talk about a subject that interests them. If you're watching this documentary, then you probably share that same interest.
Unlike some other reviewers, I'm not sure that this is a good introduction to Lovecraft. The documentary assumes that you have at least passing familiarity with each story, though it usually does give a very brief synopsis. I would have preferred that they had either gone much more in depth, with actual literary criticism (they had S.T. Joshi there, after all) or done more of a beginner's introduction.
In the end, I can't really recommend this for either beginners or fanatics. It occupies an awkward middle ground: those who have read the stories but don't really know much about Lovecraft. Thus, I recommend it mostly for the interviews. One of the interviewees is a complete ass, going on some rant about "islamofascism" in the middle of the documentary. I can't believe they left that in, yet they barely got any comments at all from John Carpenter. And this is right after going on about the Lovecraft's xenophobia and racism. Amazing.