I won't talk about the last third of the movie. I'll just say that, even though it's completely different than the two-thirds proceeding it, it's every bit as good. It involves the kind of turning point which usually breaks these kinds of movies. Body Parts pulls it off expertly.
The catch? His control over the arm is only tenuous. When curling a dumbbell, the arm lashes out of his control for no good reason. When shaving, he has another malfunction which ends up gouging his face with the razor. When he makes love to his wife and the new hand slides between her legs... well, that's about as good as suspense gets. The main character can't run away from the horror, can't hide from it in a closet. It's always there, wherever he goes.
After Fahey begins having nightmares which he believes are a direct result of the transplant, he goes to the police station and has his new fingerprints taken. It turns out the hand belonged to a recently executed death row inmate who murdered over thirty people with his bare hands. When Fahey fails to get answers from the doctor who did this to him, he seeks out others who got the killer's parts. One of the other beneficiaries is played by Brad Dourif, who's just as compelling as Fahey. In fact, the entire cast is a cut above most horror movies, which is one of the reasons it's so damn good.
I'm often annoyed when movies keep teasing what we already know, but that's because most movies feel so artificial and contrived in the way they go about generating suspense. Fortunately, there's something inherently unsettling—and thrilling—when the source of that suspense is a physical part of the hero. I'd go so far as to say it's a great movie, with good music and solid camera work. And even if the trailer was misleading about the movie as a whole, it eventually gives us exactly what was advertised.
I think it's a rare movie that, potentially, could be just as appealing to non-horror fans as it is to gore hounds. The gore, when it finally arrives, is both tasteful and utterly satisfying.