Sweet Tooth is, sadly, very underwhelming. McEwan's point is well made, but not worth the effort it takes to get there.
What are we looking at here? Freedom of thought? Power of fiction over worldly powers? The power of the spy in all his iterations? It all comes down to fiction vs. reality, in many guises. I see that and it would have made a lovely short story – it's very possible that that's on purpose and kudos to McEwan for that joke, which is actually pretty funny - but none of these themes justify sitting through twelve hours of a bland, mildly annoying, improbable protagonist and a story where every little piece of suspense goes up in nothing, despite it's highly suspenseful setting.
If this were young adult fiction, I'd get why the protagonist's obsessions have to interfere with her life, but in this context it feels petty and unbelievable. It's almost as though she walks through the world with an attitude of "my sexual satisfaction trumps all other interests, and I'm not even sorry," but she's not combining that attitude with the feminist convictions that should logically underly it. You work at MI5, and you don't think twice about starting something with your immediate superior or jumping into bed with the person you're handling? Just because you can?
The ending doesn't really change much here… it explains her lack of depth as a character in a novel, but doesn’t make the story any more relevant. It is a self-reflective ride of contemporary fiction, which is all neatly mirrored in the protagonist - which is all academically lovely, but doesn't elevate the pleasure of the story.
So the whole influence of fiction on reality, yes, sure, but in the end I am left with a piece of fiction that has sorely little influence on my reality – I feel like I am being force-fed his point in many guises, but because they are all self-contained in something that is entirely fictional if we take two steps back from it, they ultimately fail to hit home. This distinguishes Sweet Tooth from Atonement, where the reader's investment in the story is an enormous part of the reveal at the end - the fictional nature of the entire story as told by McEwan doesn't matter there, because the emotions that carry its enormous weight are very real for the reader.
In Sweet Tooth, the reader is left with nothing to feel much about. It's amusing, academically clever. Nothing more.
(In addition, the narrator intones the story with a certain downward cast that might inhibit my enjoyment of the protagonist and my willingness to believe that men would find her so irresistible. She doesn't come across as particularly lively or humorous. This might not be entirely McEwan's fault.)