If you liked the original Paranormal Activity, you'll like this one. Myself, I was not really that impressed with the original, and I liked this one quite a bit better. The mood and creepiness was pretty intense at times.
It is NOT just a remake of the original movie, as some would have you think. Yes, it uses that format, and follows it pretty closely, but does it better and takes it a few steps farther. Just because it uses the same format doesn't mean it is the same movie. That is like saying all 'Cop' movies or all 'Buddy Pictures' are the same, just because they also follow the same format.
If you saw the first movie, you will recognize everything that is happening here, but that doesn't decrease the sense of dread and hair-raising-on-the-back-of-your-neck scariness. Instead, I think it will increase your enjoyment as you watch things develop. Talk about things that go 'bump in the night!' And sometimes in the daytime, too! I thought this movie was more intense than the original, and it helps to put the original into a context, and makes more sense out of it all.
Is it a perfect movie? No, but show me a perfect movie. They all could be better in SOME way, and this is no exception, but I think it is a pretty darn good movie, and worth the time.
Oh, I should also tell you kind of "Where I'm coming from." I LOVE a good horror movie. I LIKE to be scared. But I hate slasher, torture, vicious movies or movies that get too overt about hurting people. Give us something we can feel, something we are afraid of, but don't make us sick. I think Paranormal Activity 2 is just my kind of horror movie.