Wow, this movie is taking a beating. I rather enjoyed it, but I seem to be in the vast minority here.
There will be some spoilers here, but really, the upshot of my review is that it's a 90% bog standard alien invasion B-movie.
First, the premise. Aliens invade earth, proceed to beam up people into their ships to suck out their brains and "life force". I should note, that this is absolutely no different or more cheesy than other alien B-movies, so picking Skyline apart based on that is unfair. It's not pretending to be a high brow intellectual production.
Here is also revealed the best part of the movie, which sets it apart from virtually every other alien movie, is that we know NOTHING about these aliens. No scientist figures out their language, or hacks into their mothership, or divines their intentions, or finds a trivial way to defeat them through some silly built-in weakness. There is NOTHING. They're just super-technologically advanced beings against whom humanity stands no chance. None. They come in and annihilate everything at will, not a question of how much, but how quickly.
Second, the effects. Most of the movie is special effects. As far as sets and practical effects, the budget was simply too small. Just some dude's apartment and a blue screen. But, the aliens look very cool. As always with special effects, the question is how well they will age. Sure, they look great today, but chances are they will be cheesy by the next decade, if not sooner.
The entertainment value in Skyline comes from the look and atmosphere, and the vision of the aliens themselves. We don't know who they are. They're not interested in negotiations. We don't know how they view us; do we matter, or are we just ants? What are their values? Many alien movies like to anthropomorphise their aliens. Which is boring, because they all end up being the same. They should be alien, in every sense of the word. And Skyline brings that vision a step closer.
That said, I agree with some of the more coherent reviewers here that Skyline is a couple minutes too long. It should have ended when the two protagonists get sucked into the alien ship. There, the end. Either way, it's not a happy ending, which I admire, because it takes balls. You know when you do this you're excluding most of the movie watching public. Even The Darkest Hour, which showed promise on the plot, on paper, but is far and away vastly inferior to Skyline in every way imaginable, tries to pull off a happy ending; there is a way to fight the aliens.
On the balance though, I've fund Skyline a worthy addition to my BR collection; not an Oscar contender by any means, but enjoyable for what it is. Keep expectations in line.