The most useful piece of advice on literary criticism that I've ever come across is Nabokov's dictum to identify with the author, not the characters. This book is a perfect example. If you make the mistake of identifying with O, it's all a bit bewildering. Why exactly is she interested in being blindfolded, tied up, whipped, and fucked from all angles by a bunch of people she doesn't even know? It seems bizarre and rather distasteful.
The rest of this review is in my book If Research Were Romance and Other Implausible Conjectures