I suppose I have to preface this review by saying that I am already a Glen Duncan fan, and have read all of his published novels save The Bloodstone Papers (on the reading list). At the point of my first reading of 'I, Lucifer', I had only read one other - 'Weathercock', which rocked my world and left me wanting more.
If 'Weathercock' ended up hovering around the top of my 'favourite reads' list, 'I, Lucifer' shot straight to first place. Where some of Duncan's books fall down is in the personality of the protagonist, frequently a young man musing on life after a catastrophic emotional event - they sometimes end up infuriating me with their whininess. Not so here. Lucifer has suffered, yes, has possibly been done wrong to, and yes, he complains about it; but his ultimate motive is to win over the reader, and so he narrates charmingly, persuasively and wittily.
Even while sympathising with the Devil, though, you are being shown his flaws. The more obvious ones he doesn't attempt to deny (namely, his urge to do evil), but there is more to him. This becomes more pronounced a subject towards the end of the book.
Glen Duncan's style is perhaps, as has been said, self-indulgent, but also wonderfully descriptive. He says exactly what he wants to say, sometimes in less, sometimes in more words. And what's more, what he's saying is often what a lot of us would rather ignore. He pinions human nature perfectly, using his fallen angel.