I have been a fan of Ian MacEwan since his first short story collections, First Love Last Rites and In Between the Sheets. After the excellent Atonement, which had some fun at the expense of modern writers who research everything to the nth degree and then regurgitate what they've learnt, regardless of relevance to the book, MacEwan falls into the same trap by swotting up on neurosurgery and then feeling compelled to let the reader know everything he has studied on the subject.
"Saturday Night" seems to be a repeat of the "Enduring Love" story, only less compelling. Both books feature an accident throwing characters, who otherwise would not meet, together and then exploring the unpleasant consequences which follow the chance encounter. Both also have a rational scientist type middle-aged man as the main character, confronted with emotions and situations outside the comfortably rational world of science. Both feature an irrational protagonist. Both central characters end up resolving the confrontation by realizing that rational science cannot provide a solution for everything.
While the book is extremely well written, MacEwan has also fallen into the trap of many contemporary authors (see Iain Banks) of trying to appear hip by mentioning current music etc - here we have references to Steve Earle's El Corazon, a girl sustaining a skull fracture falling out of a tree while watching Radiohead.
This is not his best book by any stretch, but compared to most modern fiction it is still pretty good.