When laid out like this, it sounds like a silly premise and I guess that it is, but both King and Carpenter are able to ground the story in the mundane at first, only gradually introducing the more fanciful elements as the story proceeds. King has always had a knack for imbuing conventional, every-day perks of modern life with something more sinister. Here, it's a car. In The Shining, it's a hotel. In Cujo, it's a dog. And so on. There's something archetypal about this sort of thing that King is able to capture, and that Carpenter is able to maintain in the adaptation.
Both versions of the story do a reasonable job portraying the superficial pleasures of teenage, suburban life. There's a cynicism that underlies this that could be obnoxious, but both King and Carpenter are able to touch on these ideas without completely drowning the story in misery. As befits most fiction, the relationships and interactions are a bit exaggerated, but not so much that you can't relate. Characters are flawed and not totally likable, but you can still empathize with them.
King's book obviously allows much more time to establish Arnie and the gradual descent he undergoes as he's driven by Christine (irony!) or, more accurately, her former owner, Roland D. LeBay. It never really drags, and King does a good job capturing the community and families involved as well as the main characters. We get a lot more about Christine's previous owner and his troubled history (before and after the car). Arnie begins to talk like him, act like him, and Dennis even notices that Arnie's signiture has changed (implying that he's sort of possessed). Christine drives around by herself, but really it appears to be LeBay's spirit that's doing the driving, and as the story progresses and Christine picks up more power, people start to hallucinate in the car, even seeing things like the rotting corpse of LeBay.
Carpenter's adaptation neatly simplifies all of this, directly imbuing the car with malevolence. It's a choice that works while still allowing the movie to hit many of the same beats as the book. Obviously much of the story is cut out and that does have an impact, particularly when it comes to the third act, which does feel rushed. Still, Carpenter is able to cleverly devise visual treatments to emphasize Christine's nature without resorting to anything particularly showy. Lots of steadicam shots, low angles, and great nighttime cinematography of headlights suddenly appearing in the darkness and so on. The car looks fantastic, and Carpenter lingers just long enough to let your mind wander. Are we, the audience, just as attracted to the car as Arnie? It's a restrained but very effective approach. The use of music on the radio in the car can be a bit on the nose, but it's a reasonable device to use for the medium and it's not overdone. The sequence where Christine rebuilds herself, which relied on practical effects, is well conceived and perfectly executed (were this made today, I'm sure the inevitable reliance on CGI wouldn't be nearly as effective).