This is a DreamWorks picture clearly made by people who don't care. Every element is dull, uninspired, and lifeless. The plot is obvious and dull; it's like reading though the TV Tropes article.
Every plot point is telegraphed in the most ham-fisted manor. The humor is flat and predictable; if you've ever seen a DreamWorks picture before, you've already seen these jokes. On occasion, the jokes work, but not nearly often enough.
The characterization is the biggest misstep; they are made from the flimsiest cardboard. In the early going, the film enjoys forgetting about the main character, instead treating us to the titular Aliens invading. For a good 10 minutes. That's fine and all, but we barely got any character establishment before cutting away; you can't just dump the main character 15 minutes into the film.
Indeed, we never really learn much about
Susan. All we know is that she has parents and that she was about to be married to some jerk. That's all.
Here's an example. In her intro scene, she goes on and on about seeing Paris. Why? We never find out why it's so important to her that she go to Paris. It just is. You would think, given how much they harp on it (not to mention the ending) that there would be a scene where she explains why it mattered. But nope; the film just doesn't care.
The supporting cast is nothing more than a set of caricatures. The supporting creatures get exactly one characteristic, and that's it. The insect guy is smart, the blob is dumb, and the Missing Link has an inferiority complex.
Here is the film's idea of "character development." After a hard-fought battle where Susan basically won single-handedly, the others are depressed. So she gives them a pep talk. There: characters developed.
The villain is probably the worst. He spouts cliche dialog ad nausium. Almost every line of dialog he has is a copy-and-paste job from a dozen other movies.
The sentiment of the story is appreciated; I get what they're trying to say. It's just that this film says it in such a boring, obvious, lumbering, overbearing, and tedious way that the message itself is irrelevant. It's just not worth the cost of having to sit through it.
Overall, it's watchable, but you can do much better for 90 minutes of your life.