The Good: I found the novel was better than the Donald Sutherland movie version in that Finney managed to create a increasing sense of tension and paranoia that didn't quite carry over to the big screen. As the story progressed I felt a definite buildup, a feeling of increased risk, that drove my need to finish the novel even though I already knew what happened.
The Bad: While the atmosphere was phenomenally construed, the aliens themselves were a little hard to grasp. Or at least in the way they were described in the novel. Had I not had the ideas put in my mind from the movie, I would have had a hard time picturing what exactly Finney was describing by the "blanks."