The first part of it was tedious. I could see Nabokov was a great writer but still, it was tedious. I struggled through first 80-90 pages and was awarded for my efforts with a brilliant second part of the book. I was actually sitting on the tube going to work, reading it and muttering to myself "Oh, brilliant, brilliant".
Hermann is such a perverse narrator. He plays with you and he is not to be trusted. One of the very few books when I felt I created a relationship with the narrator.
Don't be misled by first 50 pages - the book actually does have a plot.