The reviews will talk about whether it was scary, gory enough etc. or a huge letdown. This will probably help you more than asking people here to rate how scary it is from 1-10 as everyone gets scared for different reasons or not scared at all for different reasons.
At least with the link above you can click around and read multiple opinions and know whether it stinks or not because they have a ripe tomato to recommend something and a green one if it stinks.
I haven't seen the film myself but do go to the movies more than once a week. Right now 66% of critics on Rotten Tomatoes like the film. The critical consensus on that site was quote "Aside from its clichéd resolution, Disturbia is a tense, subtle thriller with a noteworthy performance from Shia LaBeouf." end quote from RT.
My whole beef with not seeing this film myself is that the synopsis as well as the trailer makes it look like a rip-off of Alfred Hitchcock's classic film Rear Window.
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