Stephen King was raised on flicks like this. -Flicks NOT films.
Movies like this and 'Jeepers Creepers' are "throwbacks" to the good 'ol day drive-in horror flicks. They are meant to be fun, cheaply made and hopefully: a few good scares.
Anyone looking for a theory on the human condition should pass on this creature feature because that's all this is... all it ever will be.
Stop trashing what has already deemed itself as trash. -Good, fun, trash!
If you enjoyed this I recommend: 'Jeepers Creepers' 'Jeepers Creepers 2' '30 Days of Night' 'Scarecrows'('88) 'They Live' 'Planet Terror' 'Death Proof' and 'Halloween III: Season of the Witch'

It seems to me that Stephen King's "Bachman" pen-name was a way for him to put out some of the grimmer, rawer, more mean-spirited stuff that he wanted to write without 'contaminating' his 'brand name'. If you look back at the "Bachman Books" (Running Man, The Long Walk, Roadwork, Thinner) you notice they have a sealed-in feeling of airlessness and hopelessness about them that is distinct from mainstream King. I realize that we are talking about the guy whose first novel featured a humiliated, blood-covered, emotionally crippled teenage girl slaughtering everyone at her high school prom...but mainstream King always at have characters and plot elements that leaven the grimness of the proceedings a bit, and mostly have endings that offer at least a glimpse of hope and human feeling. Bachman books are just plain mean and always end badly. (BTW, "Pet Semetary" could have easily been a Bachman book if King hadn't revealed the alias by then. And "The Dark Half" seems to be at least partially about his "Bachman" persona.)
"Thinner" was the last Bachman book, and man, with its themes of class warfare, revenge, and death by starvation, it is nasty. So it should be no surprise that the movie follows suit.
What is a surprise is that the adaptation seems to be filmed at a "TV Movie Of the Week" level of talent instead of something worthy of a theatrical release. (These days, something like this would probably go directly to DVD or cable). The makeup work and the striking motif (starving to death in the midst of plenty, a metaphor for the overfed, undernourished American middle class if there ever was one) is all that keeps you watching this misfire.
What went wrong? My first thought is that the director was going for the nasty Bachman vibe, but he also somehow sucked all the interest out of the movie with poor casting choices - the actors here (with the exception of Joe Monetegna) simply can't carry the movie. And then he squished the warmth and life out of the rest of the movie with awkward pacing and scene structure. Plus he couldn't leave the plot alone, and his changes don't really help. The script and dialog ought to work, but mostly the movie just lies there. Everything is muffled, dull, airless, and no fun to watch...with the vivid exception of the spectacle of the main character getting....thinner, and thinner, and thinner.
As other have pointed out, "Thinner" is by no means the worst King movie ever made (or even the second worst). And it does have a dreadful, compelling fascination owing to the theme and the careful makeup work. But first time viewers should approach this one with lowered expectations.