Fulci has made both good and bad films, so the fact that he (reputedly) walked out on the making of this one should make a statement about how bad it is. It was taken over by Bruno Mattei (who directed the odious Inferno dei Morti-Viventi), and it was all downhill from there.
Before I get started, let me say that I know that zombie films don't need high standards. Coming from a long history of budget films, where all you need to do is put green face paint on an actor that is capable of walking slowly, your average zombie film is not going to win any Oscars. Even so, there is a minimum that I expect, like some freakin' internal consistency.
It seems like Mattei thought up all the scenes he wanted in the film, ordered solely by the number of people left, and then shot them without regard to how each scene related to the others. For instance, even though the film supposedly takes place over a couple of days, a hotel that is fine in the beginning of the film has time to become an overgrown ruin by halfway through. Why? Because overgrown ruins are creepier. The zombies are also slaves to the needs of the scene. Sometimes they shuffle menacingly in classic zombie fashion, and sometimes they leap nimbly off of rooftops and land next to the heroes in a ninja pose. The effects were also pretty bad. In many of the fight scenes (with the more sprightly zombies), the film was blatantly speeded up to make the fight more exciting. The makeup was barely mediocre, and there were scenes where a gun would not even be aimed before you heard the bullets being fired.
The basic plot is that the military has hired some scientists to work on a biological weapon (called, I believe, "Death 1") for them, and it gets loose, causing an outbreak of zombies. Mattei tinkers with some other zombie laws, such as that these zombies can be killed like normal people (and yet men armed with machine guns and flamethrowers completely fail to fend a handful of them off). The plot is littered with cliches and scenes stolen from other movies, so don't waste your time seeing this one.