It's a brilliant, brilliant novel, IMO. Almost endlessly fascinating and disturbing. One of the brilliant things about is that you do feel sorry for Humbert, even though he's a monster, one of the most massively self-centered and nasty characters in fiction - and yet he manages to persuade you that he has a soul.
Not the least amazing thing about it is how a middle-aged Russian author so completely absorbed late 40s/early 50s American culture and slang. Lolita is herself one of the most touching characters Nabokov ever created, genuinely tragic and in a strange way heroic (think of her dignity in the last meeting she has with Humbert, when she's pregnant and a bit faded and sad - it's one of the things Dominique Swain got completely right in the otherwise only OK Adrian Lyne movie).
It's interesting that Nabokov himself thought that Humbert was a really evil and unpleasant character, and was extremely fond of Lolita; it fits the idea that he was, as some critics have said, a moralist disguised as an aesthete. Nabokov never intervenes within the book to excuse Humbert. He leaves it open whether or not Humbert ever really realises what a monster he is. Outside the book, Nabokov made it very clear that he despised Humbert, so people who say that the book is some sort of an apology for paedophilia are talking nonsense.
It's one of my favourite novels partly because it's such a seductive portrait of evil. Only Nabokov would have dared to tell the story from the point of view of someone who is, to be blunt about it, a paedophile. Unfortunately that's enough to stop some people from ever reading it, but that's their loss. Fiction should be about showing us how to empathise with people utterly unlike us; 'Lolita' certainly does that. (I write this as a fan of the book and as the father of a daughter.) Humbert is sufficiently like the rest of us to make us doubt that evil people are a different species, and that's true artistry.