A refreshing change from inane American faux horror films. The tone reminded me of Let the right one in (original version, and book), but with much more action and the added anxiety of being set on a plane in flight.
The mix of emotional narrative with action, suspense, and horror heightens the viewer's trepidation in an unsettling manner. It's not going to appeal to everyone, especially if you are going to rely on the dubbed version instead of original German with subtitles if necessary. On that note, many of the one star ratings are related to this dubbing and not the movie itself, some even appear to have not seen the same movie... or are just lazy and trolling.
If you can't understand that different cultures are going to produce different types of cinema, don't watch them.