Cujo is okay, but inconsistent and overall nothing special. This first part of the movie before the crazy rabid dog stuff starts happening is pretty boring and I wasn't particularly interested in the characters' affairs and personal drama. I was pretty much just waiting for Cujo to start attacking people.
Once he does, the film definitely starts improving. It does a reasonable job of getting across a tense atmosphere and Cujo is effectively intimidating and scary. It just takes way too long in getting there.
They did a good job of making up the dog so he looks rabid, although he looks pretty cute without the makeup.
I understand they couldn't just have a whole movie of a woman and her child fighting off a dog, but there wasn't enough meat to the story to be compelling. I haven't read the book, but I'm sure the affair plotline was significant as I've heard the movie was relatively faithful. So maybe this just wasn't enough of a story for a feature film and would have worked better as a short or a TV episode or something like that.
Also, don't expect any gore, the horror is more psychological, which isn't at all a bad thing.
I'd recommend this if you've already seen a decent amount of 80s horror and just haven't gotten around to this yet.