Not a bad movie, I'll take a shot at explaining the aliens.

Rating: Null/10 (15 August 2002)

SPOILERS!!!
There are really two stories going on here that kind of merge in the middle they are:
1. A man piecing his life together after a horrendous tragedy.
2. An alien invasion.
Weird, I know.
Story one is by far the more superior of the two. It really gets into the minutia of the emotional devastation wrought over the family. It really is along the lines of "In the Bedroom" as far as character development goes.
Story two is kind of thrown in there for a kick. The aliens could have simply been rabid racoons terrorizing the family and nothing else. It's that irrelevent to the movie. Story two just kinds of drops off and wraps up at the end. I gotta few netpicks of some user comments about story two however...
1.) Complaints about aliens not wearing clothes.
Maybe they where, or maybe they don't need them. Who knows, they're aliens. Different biology, culture, technology etc... I wouldn't stretch too much to look for similarites between them and humanity. It's like getting mad at them for not following the US Constitution.
2.) Complaints about aliens not using weapons.
Again, they're aliens, it could very well be that they prefer hand to hand combat for cultural reasons. Obviously, they can travel through space, they do have the technical prowess to create weapons.
Hell, some Native American tribes used to consider simply getting close to an enemy to touch them as an act of bravery. This doesn't mean they couldn't create an ax.
Also, maybe it wasn't a full blown invasion, just a recon mission of some sort. No point in bringing out the "big guns" when you simply want to scout out the environment and get some "local" samples. It'd also tip the natives as to what weapons you have and give them a chance to develop counter measures when the "big invasion" starts.

3.) Aliens being "human-like".
Why not? Makes for some intersting plot twists (ie. Man like figure gibson sees on the roof) and spookier video footage.
4.) Aliens being "water phobic"
I agree this is simply non-sensical at first though. It's the logical equivalent of humanity invading a planet that has a methan atmosphere. However, everyone is missing the obvious.
THEY NEED WATER BADLY.
Why else would they invade a planet that has a substance, that's deadly and toxic to them, UNLESS THEY DESPRATELY NEED THAT SUBSTANCE.
5.) Alien invasion simply ending.
I don't think it was a full blown invasion. The crop circles, probing attacks, etc... are really no different than simply marking out a territory before an invasion. That invasion could be months/years/centuries away, but it's not stupid to simply have the invasion end, its genius. The aliens get to learn from the experience and retool for round two (which may be "the big 'un"). It's actually more realistic than the "Independce Day" scenarios.