The debate will rage on: fast or slow zombies. Another debate will continue: Is 28 Days Later a zombie film. One debate won't continue though and that is whether or not it is a good horror film. It dishes out everything you;d want: high intensity suspense, gore and an astounding score. Usually when bitten by a zombie, the effect actually takes time. 28 Days plays it out that if you get so much as a drop of blood on you, the change is almost instantaneous. Something never quite mined from the undead/infected genre, or at least not as successfully until 2002.
It could have been a perfect movie had it not been for some of the all too influenced Day of the Dead ending. That being said, it's easily Boyle's best work since Trainspotting. It's works like these that are proof that the genre of horror isn't entirely dead and the end is certainly not fucking nigh.