I liked it a lot. M.Night's best movie imo.
Love the atmosphere, the music, the "scare" scenes actually scared me lol. Only complaint is that it drags in some parts.
Also really, really liked how he tied in the concept of everything DOES happen for a reason....even down to the little girl not liking the water, therefore she left water glasses all over the house, setting up the way to kill that alien...and the little boy having asthma prevented him from breathing the poison. And how all those little things, when Mel's character realized them, helped restore his faith. That was great. Also gave the title "Signs" a dual meaning.