Which is to say, it's excellent. It maintains a high pitch of tension through the whole 100 minutes, something that a lot of horror doesn't manage. All too often, I find, a movie will start out scary enough, but then by some point about half way through it's effectively turned into an action movie.
This happens when the nature of the monster/killer/zombies have been revealed and all the protagonists have to do is fight it out - with the uncertainty gone, the horror goes, too. Without giving too much away, The Crazies avoids this trap. (The last great horror movie I saw, Paranormal Activity, does too, although in a very different way).
Of course the real reason I liked this movie is that it's got some neuroscience. The Crazies is (spoilers) about an engineered virus that infects the brain. Early symptoms include fever, blank stares, flattened emotions and stereotypies. This then progresses, over the course of about 48 hours, to psychopathic aggression, at least in some cases, although other victims just become confused. The "crazies" are somewhat like zombies - they have a Zombie Spectrum Disorder, one might say - but they retain enough of their personality and intelligence to be capable of much more elaborate and calculating violence than the average braaaaaaains-muncher, which is what makes them so disturbing.
Could a virus do that? Rabies, notoriously, causes aggression in animals and humans, although the incubation period is weeks rather than days, and aggression is only one of many neurological symptoms of the disease. But maybe an engineered virus could achieve a more specific effect if it was able to selectively infect the area of the brain reported on in this rather scary paper:
The authors report a patient with advanced PD, successfully treated by bilateral stimulation of the subthalamic nucleus, who developed acute transient aggressive behavior during intraoperative electrical test stimulation. The electrode responsible for this abnormal behavior was located within the lateral part of the posteromedial hypothalamic region (triangle of Sano). The authors suggest that affect can be dramatically modulated by the selective manipulation of deep brain structures.