Forget the demographics (unless accompanied by kids), forget the reviews, and forget what it looks like. I'm an AVID movie goer, and I see just about every mainstream film (and some foreign) that comes down the chute worth seeing.
Now don't misunderstand me, I've had guys go see 'Dumb and Dumber', a superb film only to come back and tell me it was too dumb for them and they couldn't get down to it's level. Well the movie tells you twice in the title, that it's dumb so that's your fault. This is a horror/terror movie with marvelous concept and great FX, that is intentionally funny at times and very well acted (Patrick Wilson - Night Owl from Watchmen, and Lynch from the A-Team; and up and coming Rose Byrne from Knowing, Get Him To the Greek, Marie Antoinette, and Dr. Moira MacTaggart in the next X-men). No it won't be nominated for anything, and of course there are things I'd have done differently (as an aspiring screenwriter) but it's a great movie.
And I'll give each of you my rarely seen money back gaurantee. If you go see Insidious and honestly don't like it, just post a pic of your stub in this thread, and I'll refund your ticket price. IT'S THAT GOOD!
If you know this paraphrase you'll love the movie, if not don't bother with it:
"No can be told what Insidious is. You have to see it for yourself."