I really liked the more scientific explanations of the zombies in this one. Romero set it up so that, as each movie progresses, the understanding of zombies greatly increases. In between Night... and Dawn... is the greatest difference, I think, but Day still has much to add to the revelations, especially the discoveries of "Dr. Frankenstein." No, Romero wasn't that corny, that doctor is given that as a nickname by the soldiers because he delights in dissecting the zombies in his pursuits for a solution. Also, the character of Bub, who was a zombie trained to be "good," has interesting implications. And perhaps my favorite part of all is the opening scene, where the long-absent sound of a human voice evokes an inundation of zombies that clog all of the city streets...to understand, you'd have to see it. Great scene, though, I think.
Unfortunately, this movie was not nearly a match for the original script, which can be found at the Homepage of the Dead. The characters, with the exception of the helicopter pilots, were not especially likeable. Well, I'll give Rhodes some credit for being a good bad guy, but overall, the characters were kinda bland. I wasn't especially impressed by some of the zombies, either; I don't know how the hell a football player made it all the way out to the jungle? Whatever. That's not important. I'd say that, although the ideas Romero had with this were interesting, it got kinda boring and bland after he was forced to nix his original script for budget purposes and make this in its stead.
However, I still recommend this movie. Though some parts lie perhaps inferior to the others in the trilogy, some parts are likely among the best of all three. If you like zombies, you'll like Romero and you'll like Day of the Dead.