I'd heard from multiple sources that this was one of those books you either loved or hated. I'd also heard that it was either a tough slog (it is rather lengthy) or that you get to a certain part about 2/3 in and throw up your hands. I also had the experience of not enjoying her previous debut novel (The Secret History). This is what I started with.
I agree with all the sentiments. It was definitely a tough slog in places. It's not giving much away to say that there is a lot, lot, lot of druggie and alcoholic and general bad behavior all around in this novel. I don't live my life this way, so it got exasperating. While we understand the reasons why, for the most part, it's hard not to want to reach into the pages, grasp Theo by the shoulders (or the neck!) and try to shake some sense into him. Because it just goes on too lengthily. Or so it feels at the time. When you're done, you understand how all the pieces come together. But that doesn't excuse the deadly dull parts.
I did also get to a certain part and throw up my hands. I was invested enough in the story to not also throw the book across the room and not retrieve it. However, I can see how some would. To them I would say that if you just give it another 75 pages it will pay off for you, and that the final 50 pages are some of the best writing - and the best summary (for lack of a better word) of an entire story - that I've ever read.
Therefore, to everyone I say: stick with it, it will pay off with all sorts of dividends.