Steven Spielberg's War of the Worlds is a pretty tense affair. The director knows how to lay on the suspense and he certainly applies that knowledge liberally in the film. It's a good thing too, because when he allows a short breather, your mind immediately starts asking questions that can only have embarrassingly illogical answers.
Luckily, Spielberg's version of the infamous H.G. Wells novel focuses on one character, not the big picture of the story. This relegates the aliens in the film to a MacGuffin, a mostly unexplained excuse to place pressure on the protagonist Ray Ferrier (played competently by Tom Cruise). In this respect, it resembles M. Night Shyamalan's Signs more than other recent big budget disaster films like Independence Day. Its pacing and relentless tension make the film feel more like horror than science fiction. Unfortunately, there's enough pseudo-explanations and speculations about the aliens to strain the suspension of disbelief that is required for this film to work. I've found that I generally have more movie-going goodwill than others (i.e. letting art be art), so I didn't mind the lack of details and even some of the odd quirky logic that seems to drive the plot, which really focuses on the aforementioned Ray's relationship with his kids (and not the aliens). Ultimately, there's nothing special about the story, but in the hands of someone as proficient as Speilberg, it works well enough for me. It's visually impressive and quite intense.
Besides, it's not like the concept itself makes all that much sense. In 1898, Wells' novel was probably seen as somewhat realistic, though the Martians-as-metaphor themes didn't escape anyone. In 1938, Orson Welles's infamous radio broadcast of the story scared the hell out of listeners who thought that an actual invasion was occurring. Today, the concept of an advanced alien civilization invading earth has lost much of its edge, perhaps because we understand the science of such a scenario much better than we used to. If you're able to put aside the nagging questions, it still holds a certain metaphorical value, but even that is starting to get a little old.
No explicit motivation is attributed to the aliens in Spielberg's film, but in other stories it generally comes down to the aliens' lust for resources ("They're like locusts. They're moving from planet to planet... their whole civilization. After they've consumed every natural resource they move on..."). This, of course, makes no sense.
Space is big. Huge. From what we know of life in the universe, it appears to be quite rare and extremely spread out. Travel between civilizations may be possible due to something exotic like a wormhole or faster-than-light travel, but even if that were possible (and that's a big if), traversing the distances involved in the usually huge and powerful alien craft is still bound to expend massive amounts of energy. And for what? Resources? What kinds of resources? Usually "resources" is code for energy, but that doesn't make much sense to me. They'd have to have found something workable (perhaps fusion) just to make the trip to Earth, right? In the miniseries V the aliens are after water, which is an impressively ignorant motivation (hydrogen and oxygen are among the universe's most abundant elements and water itself has been observed all over our galaxy). Perhaps the combination of water, mineral resources, a temperate climate, a protective and varied atmosphere, animal and plant life, and relatively stable ecosystems would make Earth a little more attractive.
What else makes Earth so special? There would have to be some sort of resource we have that most other planets don't. Again, Earth is one of the rare planets capable of supporting life, but we can infer that they're not looking for life itself (their first acts invariably include an attempt to exterminate all life they come accross. In War of the Worlds, the Alien tripods start by vaporizing every human they see. Later in the film, we see them sort of "eating" humans. This is a somewhat muddled message, to say the least). And whatever this resource is, it would have to justify risking a war with an indigenous intelligent life form. Granted, we probably wouldn't stand much of a chance against their superior technology, but at the very least, our extermination would require the expenditure of yet more energy (further discrediting the notion that what the aliens are after is an energy source). Plus, it's not like we've left the planet alone - we're busy using up the resources ourselves. Also, while our weapons may be no match for alien defenses, they'd be quite sufficent to destroy much of the planet's surface out of spite, rendering the alien invasion moot.
The only thing that even approaches making any sort of sense is that they want Earth as a new home for themselves. As one of the few planets capable of supporting life, I suppose it could be valuable in that respect. Indeed, in Wells' novel, the Martians attacked earth because their planet was dying. Spielberg's film seems determined to kinda-sorta keep true to the novel, except that the aliens appear to have planned this countless years ago, which makes it seem less likely. But again, why risk invading an already inhabited planet? Some stories have emphasized that the aliens were doing their equivalent of terraforming (this is implied in War of the Worlds when Ray looks out over a bizarrely changed landscape filled with red weeds), which is a good idea, but it still doesn't explain why Earth would be a target. From all appearances, there are plenty of empty planets out there...
So the concept itself is a bit tired to start with. Movies that aren't explicit invasions involving a civilization like our own fare a little better. Alien & Aliens do a good job of this, as have several other films.
In any case, War of the Worlds is still a reasonably good watch, so long as you don't mind the lack of scientific rigor. It's a visually impressive film, with a number of sequences that stand out. And he really doesn't give you all that much time to think about all the flaws...