Excellent. On its most superficial level its an ‘alien invasion’ story but far more than this. It has vividly drawn characters whose reactions to their experiences show development and perception. The ending has a degree of ambiguity and is not a warm and fuzzy one.
Also liked the way that there is not an elaborate explanation/backstory for the event (the arrival of a slowly-expanding ‘shimmer’ in which extreme genetic changes occur) but it is just accepted and the characters (and the viewer) simply have to deal with it. The SFX are there totally to hep the story and never for their own sake.