James Wan is one of those divisive directors but I think he has a pretty good lock on making original horror. Despite the fact that his inventions become boorish clones of the original after a few movies (looking at you, Saw), he has some great concepts and a keen sense of what would make a horrific visual.
Insidious follows a family who appear to be living in a haunted house due to the weird stuff going on around the couple (Patrick Wilson and Rose Byrne) and their son. Refreshingly, they move out of the creepy old house immediately to get away from the slamming doors and nightmares but, of course, whatever is haunting them is following them as well. There are some nice creepy moments after the family move where apparent ghosts show up and vanish just as quickly (I am thinking of a scene with Byrne and a young boy dressed like a Newsie). Eventually, they call in experts who tell them that there is a malevolent force looking to take over their son's body. The movie becomes a cross between the Exorcist and the Sixth Sense with the family and the demonologists working to keep the boy safe from bodily invasion in the real world while the father searches for his spirit in a foggy outland called The Further.
The Darth Maul looking villain, and his reliance on Tiny Tim's Tiptoe through the Tulips, are sufficiently creepy. There are some good shots with the demon sort of over the kid's shoulder looking like a baroque depiction of nightmares. This movie also gets a second life if you watch the excellent documentary The Nightmare, about sleep paralysis. Apparently, the movie overlaps with the real phenomenon pretty well.
I have never seen parts 2, 3 and 4 but I am in no hurry to. Hollywood has never met a decent idea it can't run into the ground. As a standalone movie, I would give it a B.