Highsmith was a very talented writer. This was her first novel but does not much feel like a beginner's effort.
The best thing about the novel is Bruno. He is a brilliant invention. Guy, OTOH, becomes irritating. The middle of the novel becomes a little tedious, with his constant flip-flopping, over, well, everything. He is an emotional basket case, which of course is the key to the plot. It would not work otherwise. The ends of both men are rather a disappointment. Particularly Bruno. He deserved better.
Hitchcock and Chandler took the more conventional line in the movie. I haven't seen that in years, but I think it worked better. I remember Robert Walker was brilliant as Bruno.
Strange how movies have been so kind to Highsmith, in an awkward way: I love Minghella's Talented Mr. Ripley. It moved me to read the book, and I was quite disappointed there: again, her writing is flawless, but the story disappoints--all she seems to care about is the mechanics of someone getting away with murder and living happily ever after. The movie added many more layers of complexity to the main characters, and even some of the minor ones.
Oddly, compared to her Talented Ripley, Highsmith is actually more ambitious with Strangers on a Train. The novel is, psychologically, very deep. But perhaps experience taught her to give the audience more of what they wanted, and less of her own vision.