The idea, the story, and the script, are all excellent - in principle.

Rating: Null/10 (15 March 2003)

Shyamalan likes the kind of surprise that makes us reinterpret what we've just seen, but he does so in a different way here: he makes us re-evaluate what the film's fundamentally about.
Spoilers follow, throughout my review:
It's not about the crop circles, and it's not about aliens. It's about God. We think we'll be offered clues throughout, and we are; but they're not clues about what causes the crop circles (Shyamalan simply TELLS us what causes them, as soon as he can), nor are they clues about what the aliens are like, what their plan is, or anything like that (again, what Shyamalan thinks we need to know on this topic, which isn't much, he simply TELLS us, without bothering to drop hints first). They're clues which allow us to deduce (a) that there is a god, and (b) ITS plans and intentions. The twist at the end is that Graham's take on Christianity was right all along.
Some people gagged at this point, as though theism was being forced down their throats. I don't think it was. Throughout his last three films Shyamalan has asked us to make-believe in ghosts, superheroes, and hostile aliens that prepare for their assault with crop circles; why can't gods simply be more of the same? The correct way to view "Signs" is as an apparent work of science fiction that turns out to be supernatural fantasy.
You'll note, by the way, that the God in this film is not so nice as all that. He looks out for this family, but in sly ways: he prevents the boy from inhaling poison gas, but by giving him asthma; he secures a supply of ready water all over the house, but by giving the girl a neurosis. And he looks out for THIS family. Why do they in particular get let off? I think it's because the father is a priest - a special servant. If he'd been a Hindu, or an atheist, or even (I suspect) a mere rank-and-file Judeo-Christian, that would have simply been tough luck.
Anyway, I think it's a splendid idea. (And you must admit the title is a clever one.) So why doesn't it work?
I hate to say it, but while Shyamalan's earnest approach to ghosts and superheroes enabled him to make these things plausible, he manifestly fails... not with God, but with the aliens; indeed, with everything preceding the final scene. But the aliens and crop circles were just red herrings, right? Yes, but for a red herring to divert our attention properly it must be diverting. I think most of us sensed, with the crop circles and aliens and impending invasion, that Shyamalan's heart wasn't in it; the bulk of the film was simply dull, and the alien story all too obviously wasn't going anywhere.
The science fiction bit has to work AS SCIENCE FICTION; the rug has to be firmly woven beneath our feet before it can be pulled out from under us. Unfortunately the rug is threadbare. Nobody could watch "Signs" without choking on the implausibility. The aliens dissolve when they come into contact with water. A single glass of the stuff can be deadly. This is silly enough, but then they come HERE, of all places. Forget the oceans, forget streams, forget little puddles in the ground (I'm sure at least one of their scouts must have stepped in one of these, or at the very least brushed against a plant covered with dew), forget the fact that they are visiting CROPS, which are irrigated; haven't they noticed that, on Earth, it RAINS? They must have been sitting in the sky (in the CLOUDY sky, as we see) plotting their invasion for the one three-day period in the entire history of humanity in which it didn't rain anywhere. And for what? To harvest animals that are primarily MADE of water.
These aliens don't bring weapons, they do nothing to disrupt our electricity or communications; in fact, everything about them is laughable.
And the humans aren't that much better. Maybe I'm over-estimating the English-speaking world's mass media, but surely a group of crop circles would not be treated as the kind of huge story that disrupts programs on all channels simultaneously, and even if it would be, how could the sight of bent stalks of corn possibly create mass panic? And even if this is what WOULD happen, it's not what I want to see. I want to see at least one of the main characters behaving intelligently. Is this too much to ask?
Shyamalan is a first-rate director and the script must have looked good on paper. It must have seemed not to matter that so much of it was half-baked. It matters a lot.