I've said in the past that David Cronenberg's version of The Fly is a masterpiece. There is not one single line of bad dialogue, not one bad actor, and not one scene not important to the plot in the entire film. The Fly II isn't the astounding train wreck some have made it out to be, but it is a lackluster sequel that is often ludicrously boring.
Granted, the visual effects and gore are impressive, particularly the chap who gets his face melted by fly vomit, but these effects don't manage to inspire the same amount of revulsion the original Fly did. It's just a bunch of flashy splatter effects, complete with the odd makeup or creature effect, that really aren't any more repulsive or horrific than a Dead Teenager Movie like Friday the 13th.
Speaking of, The Fly II might as well be considered a Dead Dickweed Movie, because almost none of the characters are fleshed out beyond being a bunch of absolute wads, and we have to put up with them for over an hour before the main character transforms into a fly and offs them. The plot is rather thin-- Brundle's son is born, son grows up into a fly, tries to find a cure. For a film that's 10 minutes longer than the original, the plot is too simple to be carried that long.
Whereas the original Fly kept me riveted to the screen by throwing in plenty of repulsive effects (the fingernails scene is always a favourite) in between intelligent characterization, The Fly II is nothing more than a vehicle for gore. Considering the vast majority of it doesn't show up until the last act, the film is immensely boring throughout. Martin's transformation is particularly unimpressive; even a scene where he pulls one of his eyes out to reveal an insect eye fails to impress.
The Fly II isn't horrible, but it is quite dull and filled with unlikeable characters that only make the film even more of a chore to watch. As a sequel to a Cronenberg movie, though, it could have been much worse.