This movie is worth the time, effort, and expense of seeing it, once anyway. I've seen it twice and just might go for a third viewing, while it's still in the theaters. If you need to know what you're getting into first, have a look at the Wikipedia entry.
I will say that the strength of this film has nothing to do with it being a realistic portrayal of what an alien encounter would be like. The aliens, with two exceptions, seem incapable of creating the sort of technology necessary for space flight, or even of operating the space craft on which they arrive; they're quarrelsome and their appetite seems insatiable. You might even decide it's likely they ate most of the officers on the ship in a sort of hunger-driven mutiny.
But if the aliens are implausible, the story which plays out in the context created by their arrival is a gem, beginning with the choice of South Africa as a location, and the choice of an Afrikaner as the lead character who is exposed to a fluid of alien manufacture, with the result that he begins to transform into an alien. It's a vehicle rife with possibilities.
With any luck, there'll be a sequel, in about three years. (By the way, if the producers need ideas for that sequel, I have some...)