The parts I read that I could directly relate to from my own upbringing (a mix of Middle and mid-to-upper Prole, between the two sides of my family, with my own strong coating of Middle and accidentally-adopted Upper-Middle thinly painted on top) were very accurate and insightful, even when funny. I'm inclined to believe very little of it is manufactured or even exaggerated for entertainment value, despite the wit of the book and the way it sometimes reads as jokey, even when I know for a fact it's relating plain truth.

[EDIT] That is, I wouldn't take it as a straightforward academic effort or something like that, but I wouldn't dismiss it because it happens to be entertaining. Not one thing I had first-hand knowledge of in it make me go "oh come on, that's not quite right". I'd guess if it contains such, it's a very small portion of the book.