This blog is, among other things (or inter alia as we lawyers say), your go-to resource for Zadie Smith commentary, so it is incumbent on me to report on "NW". My advice is to read Adam Mars-Jones on the topic and then my further notes below.
First, there is a lot that is good in the book. Smith's greatest skill, it seems to me, is in short passages. She can excel as a comic novelist, and jokes and funny scenes have to be short: you can't have funny structure (does dramatic irony make you laugh?).
The funny bits in 'NW' are fewer and further between than in 'White Teeth' or 'On Beauty', but there are plenty of moments that are good. I liked, for example, the description of a barrister's clerk: "As he smiled, the port-wine stain around his left eye crinkled horribly. Another of Polly's clever phrases: 'whole personality constructed around a stain'." It is a clever phrase, but it also struck me because it is unusual nowadays for a facial disfigurement to be the mark of a bad person - and particularly interesting given that this is a book which deals extensively with the fact that the colour of people's skin makes a difference to their lives.
Second, despite my earlier worries, I am pleased to say that there is no unrealistic old white man being racist for no reason. The closest is a painfully sensitive old lefty who carefully explains the meaning of a black man's 'bare' and 'long' in what I found to be a slightly cringey passage. He was potentially a good borderline comic character, similar to Annie (see below), but didn't quite spring to life.
That passage takes place in the best section of the book, the middle one, the Felix meat in the Leah-Keisha sandwich. This section is quite different from the extended outer sections: it takes one day - a particularly important day - in the life of one man and follows him and the people he meets. It contains the lively character of Annie (whom Mars-Jones also likes) a posh woman with a rackety life living in squalor. Perhaps the best achievement of this section is to make us sympathetic to Felix, an inarticulate young man who is not an immediately appealing character.
The other sections concern Leah and Keisha. They are, in the main, pretty convincing and recognisable people. (Leah is a bit wet, but plausibly so.) The main problem with them is that they are given hard to believe additional character traits which are necessary for the story to have any movement but which do not fit with the plausible accounts of their everyday lives and characters. They are both sexed up characters: in Keisha's case that is the character trait that takes her well beyond the bounds of plausibility; in Leah's case, for no reason that I could see.
Mars-Jones picks up on some wobbles in matters of detail and I felt a few of these too. Keisha is a barrister and some of the details about this are wrong in important ways (e.g. about money). One mustn't be too picky, but you'd be irritated too if a plot development happened because someone got turned away from an NHS hospital because they had lost their National Insurance numbercard. A few other examples: a British new reader would never say "The notorious Garvey House project" instead of "estate" and Smith must know that at some level; the marijuana use feels quite American; when middle class professionals are having a party in Notting Hill to look down on the carnival, do they allow smoking inside the flat?; the cause of the break between Leah and Keisha is unrealistic (can you imagine Irie Jones in that situation?); the posh young chap that Felix meets might well have aged parents who have a flat in London - but would that flat really be in Mayfair?
Moreover, there is the problem of the ending. One shouldn't give away the end of a book, but all I can say is 'NW' has the same problem in this regard as 'On Beauty'.
On re-reading the list above, it sounds like as if I am saying that it is not a good book. That's not right. But it is certainly not a necessary book: Mars-Jones may well be right that it is different exercises (one on Felix and one on Leah-Keisha) uncomfortably spliced together.
One impression I got from 'NW' is that Smith does not have a way in to the adult world of NW London: when, as adults, Keisha's sister tells her that she doesn't care about Keisha's world, we are led to believe that there is a world for Keisha's sister, but we don't know what it is. We only know Keisha's world, the world of the bright girl who leaves Willesden for university and dinner parties. Mars-Jones writes that "NW even abuts on the territory of the "Hampstead novel"": yes, and that, perhaps, is what Smith should write next.