Let me preface this by saying that I love William Gibson. I think he is a phenomenal writer who is wonderfully intelligent and imaginative. In every one of his books, he spins fanciful, thought-provoking yarns that are utterly absorbing and linger long after the last page has been turned.
So if this is true about Virtual Light (and it is), why three stars? Well, unfortunately VL felt to me like Gibson spent a lot more time worrying about some of the really neat ideas in the book (the homeless community on the Golden Gate Bridge, which was wonderfully described, the Costa Rican data havens, the TV Christian cult, etc.) than about the story.
Several of the characters felt quite underdeveloped, a few even unnecessary. This is not uncommon in Sci-Fi, even in Gibson (though his characters are usually very good, and several here are, too), but here it felt like it detracted from the story significantly rather than being a minor nuisance. Additionally, the plot, though interesting, didn't actually go far until the end of the story. Things you might expect to happen in the first 100 pages weren't happening until 250, and the horribly deus ex machina ending occurred so quickly that I could hardly believe the book was over. Not that what Gibson did in the end was bad, necessarily (minus the "divine" intervention that allowed it to happen). It's just that he took 100 pages worth of plot and condensed them into about 10.
Having said all that, though, the book wasn't that bad. I was very absorbed in it while I was reading, and almost all of the ideas in the story were very interesting. However, I'm glad this wasn't the first or even the third Gibson novel I read. I'd recommend you start with Neuromancer or his new one, Pattern Recognition, if you are new to Gibson's writing. If you aren't, this is still a worthwhile read, as long as you can forgive its flaws.