No one expects Return of the Fly to be any good. For one thing, no one expected The Fly to be any good, although it surprised a lot of people. The reason for a sequel was obvious ... The Fly grossed $3 million on a $700k budget. It was clear that there would be no point in making a sequel without Vincent Price, and when he saw the first draft of the script, he was impressed and signed on.
But this wasn't like The Terminator, where James Cameron showed he could make money on a budget of $6.4 million and so spent more than $100 million on the sequel. No, the powers that be at 20th Century Fox decided that Vincent Price should be good enough to make a profit. So the script was revised to make the film cheaper (too late, I guess, for Price to opt out). No one other than Price returned from the first film. The Fly was in color, but Return of the Fly was in black-and-white. And when Return of the Fly was released, it was placed on a double-bill with The Alligator People.
There were the usual "let's laugh at this cheap movie" things. The sequel took place 15-20 years after the first, and it was written so that the sets from The Fly could be used again. There was no real attempt to make the film look like time had passed ... Price didn't look any different, clothes and cars were the same. Brett Halsey, a handsome fellow and not a bad actor, played the boy from the first movie. And most of the plot was just a remake, rather than a sequel, to the first, i.e. man gets caught in transporter with a house fly.
And the Fly Head on top of Halsey (to be more accurate, on top of a stunt man) looked ludicrous, a real problem because we saw much more of the head than we had in the original. Not to mention Halsey had a Fly Head, a Fly Hand, and a Fly Foot, but when we saw the little fly of "Help meeee!" fame, it had Halsey's head but its own claw and foot.
Yet somehow, it works on a basic level. There's an attempt at a plot involving skullduggery, and really, no matter how cheap, there's something icky about becoming part man, part fly.
But I don't want to go too far. It's not very good, and there's no real reason to watch it as long as The Fly is out there. 5/10.