Well at least it's short. It's true that if you read just a few sentences you can appreciate the prose style. But after 50 pages you might wonder why the author needed to spend that many pages just showing an aimless couple wandering around on vacation. None of that prose adds up to a thing.
Around page 50 they meet a mysterious man. It's all very crassly done, like something written by a student. No character feels remotely real.
Then for forty pages the main characters wonder about this mysterious new friend and his wife or girlfriend or whatever she is. Forty more pages to yawn through.
Then at page 90 an odd discovery, and then the bizarre, unrealistic, unbelievable, and gross, ending. An ending that has nothing to do with anything really. It's just very sick and violent. It comes from nowhere and leaves you dissatisfied.
I suppose Ian McEwan thought he was making some point, something about power relationships (oh very trendy!), but it's pretty thin. I've noticed that several bad authors do the following: they take a wisp of a philosophical idea that they haven't really thought out, and so they know they could never really write an essay or a non-fiction book about it. The ideas are too thin and incomplete. But then they hit on a brainstorm. They can write a novel that puts forwad this philosophical idea and they'll be forgiven for not having thought it through because, after all, it's only a story. Don Dellilo's "White Noise" has been accused of this. Italo Calvino makes a living off of it as does Mulan Kundera. Anyway, the consequence is a very manipulative plot-based novel or worse, a bad essay disguised as a novel, with little to no character development.
That is exactly the case with "The Comfort of Strangers". Let's put it this way, if Ian McEwan didn't already have a huge reputation he never could have gotten this published.