Although it's more sci-fi than horror, a film I really enjoyed that was slaughtered by critics is Skyline. The plot centers around a group of twentysomethings trapped in a high-rise condo in LA, meeting up with other survivors, trying to either hide and escape from brutal alien invaders sweeping up any and all humans they can find to use for some very nasty purposes. The horror element is strong and, at least to me, terrifying.
Gerard