I rather liked INSIDIOUS, and it was one of the few times I wished that they had been given a more substantial budget for the climax, where all the tension and unease was mostly deflated due to the money not being up to par with the ambitions of James Wan and Leigh Whannell. But the acting was good, the story was good, and though INSIDIOUS doesn't hit the high points of a movie like POLTERGEIST, the point is made - you can make an effective horror film with a small budget and with a solid base of story and script.
INSIDIOUS made a terrific profit though, so my gripes about the ending probably won't hold too much weight, as according to the Hollywood Reporter Whannell and Wan are in talks to make a follow-up. It's unclear whether any of the characters or cast would return - at the end of the first film Patrick Wilson's character was in a Very Bad Place, and although I'd like to see more from those characters what I'd really like to see is more of an exploration of The Further and that world. INSIDIOUS had a low budget and made a ton of money, so I'd imagine the budget for the sequel won't be too much more, but an excursion into the Further to find out more about the demons there? Yeah, I'm all for that.