Ashfall works where most other disaster film's do not because it mixes the basic premise is a way that's novel and exciting.
This might best be described as a disaster film that's also a military thriller, with a great human interest back story. In terms of pacing, it starts off slowly but quickly gains momentum, adding in layers of excitement and varied action, as it moves to its very emotional climax.
Acting is excellent with characters who have real depth and a story to tell. Its worth mentioning that some of the humour might seem a bit odd and a little slapstick, to European audiences. Its moderately reminiscent of the kind of humour that you find in Hong Kong action flicks, Its not to my taste but then this is a Korean film, pitched at Korean viewers.
My only real complaint with Ashfall is the story is a little too complex at times and deviates in different directions, a little too much. It can be hard to keep up with who's doing what to who and why.
That said, Ashfall is still a hell of a lot better than the basic disaster movies, that lazily reply on exposition and special effects, to carry them.
A well deserved 7/10 from me.