Alien invasion sci-fi turns on the quality of a) the explanation of what the aliens want on Earth, and b) the explanation of why they haven't taken it already. A classically bad example of both is War of the Worlds, where these "vast, cool intellects" can't figure out a) how to stake out some lebensraum that isn't already occupied, or b) spend all those jillions of planning years figuring out some antibiotics, or even decent hazmat gear. Not to mention, why spend all the effort to bury deadly mecha all over the place when they could have just USED THEM in the first place.

I'm deeply conflicted over _Signs_, which was *not* meant as a sci-fi movie but rather as a suspense film with a sci-fi idea initiating the tension. As a suspense film, Signs delivered some very good stuff. That said, the water-soluble aliens were so egregiously suck-ass from a sci-fi standpoint that must confess I've never tried to watch the film twice.