Not very good. The first Paranormal Activity was good because it was new and fresh… and actually creepy/scary. By #3 you pretty much know what is going to happen… so you end up sitting through long stretches of “normal” activity waiting for the “paranormal” activity. Yes, I jumped a few times… yes, I had the lights turned off and watched it in a dark room so it was a little creepy… enough to earn it a 5-. If you liked the first one in the series, you won’t be blown away by this one, but may enjoy it a little.
My wife rated it a 0/10. She did not care for it at all and said she felt she wasted 1.5 hrs of her life watching it. Not a ringing endorsement.
I say skip this one. If you are like me and just wanted to see for yourself (because you like this type of movie and liked the first one or two) go for it. Let me know how you liked it by commenting here.