First off, I will say that I am a pretty devoted romance reader, and you can find plenty of books with similar plot point, though not all. Bondage? Sure. Super sexy 27-year-old billionaires? Easy. Naive virgin heroines? Everywhere. I, as well as pretty much everyone else, am confused as to what about this particular series is so appealing to such large numbers of people. While obviously not the main reason, the covers certainly help. Your average person wouldn't be caught dead with a romance clinch cover within a ten foot radius of them. This trilogy has fabulous covers. Simple, yet elegant.
As far as the writing goes, this series could really have used a good editing. Or two. The writing doesn't always sound all that natural, and especially since this is in first person, that can be jarring. Also, I think it might have been better if it were a bit shorter. Fanfiction is all well and good, but it is also not often well edited, and it shows.
Yes, the author is British, so yes, I can forgive a few Britishisms sprinkled throughout. I'm sure and English person looking through an American-written book set in England would find all sorts of problems. But many people find this to be a huge stumbling block to their enjoyment. Even more so if they have any familiarity with Seattle, as the author takes major liberties with her geography.
Also, this book was in first person. I can, occasionally, enjoy a book in first person (Sunshine by Robin McKinley being one I can immediately bring to mind), but generally I like to see a bit more of the other characters, not just the narrator and how she interprets the other characters. I mean, it's pretty obvious (at first, anyway) what she sees in a young, hot billionaire, but what does he see in her? Also, her talking to herself/conscience/inner goddess could get a little annoying.
Finally, the heroine was a door mat. She pretty much let the hero walk all over her, do what he wanted, set his conditions and whatever, and she went along with it. If someone were to give me a set of ridiculous conditions to follow before I had sex with them, and then went on to show me their "play room" when I'd hardly done more than kiss someone before, you could bet I would not be sticking around, sexy billionaire or not.
I really did try to get into this book. I usually do try with popular books, but all too often I am disappointed. Despite my excitement at seeing a romance novel this popular, I did not particularly enjoy it. However, you very well might. There is obviously some reason it is so popular, obscure though it may be, and it might just be the book for you. (If you're looking for good sex scenes, though, this is not really the place. Yes, it's kinky, but kinky does not necessarily equal good.)