This is a very long book, and it took me a very long time to read (two weeks) compared to the 2-3 hours it usually takes me to read a middle grade or YA novel. The point of view shifts back and forth between Bartimaeus, whose witty first-person narration includes lots of references to historical events and footnotes on nearly every page, and Nathaniel, whose chapters are told in the third person. There is a lot of information to take in about the world of this story - alternate history, the complex procedures for summoning demons, the different types of demons and their different powers, the rules for naming wizards, etc. - but I didn't have trouble catching on. Everything is well-established and revealed as needed in the text. From a purely structural standpoint, the book is solid, and the characters are interesting, even if they are not likable.
The main thing that struck me about this book, though, is how dark it is, and how there is no sense of good. The magicians are selfish, conceited, and opportunistic, and the demons are violent, amoral, and cruel. Underwood's wife and one of Nathaniel's tutors (also a woman) are the only kind characters in the entire book, and though Nathaniel is devoted to them, his attempts at showing any love toward them are immediately condemned as immature and useless. Most books of this type highlight the struggle between good and evil, but this one focuses instead on the continual struggles between different types of evil. I guess parts of the book were entertaining, and certainly the sections written in Bartimaeus's voice are clever and fun to read, but I never felt fully comfortable with the idea of indulging in the evil thoughts of these characters without so much as a glimmer of hope for their conversion to the good. I would have serious reservations about handing this book to my own children, not only because of the lack of hope and beauty, but also because of the chance that they might try to emulate Nathaniel's actions. (As Simcha Fischer says, "[Satan] doesn't care if you are kidding or not when you call him by name." I don't think playing around with pentacles and such is a good idea.)
I do think the story could be read as a cautionary tale about the dangers of magic, and the way history would be forever altered if men were allowed to accomplish their every whim by summoning all-powerful beings to their aid, but few teens are going to think of it that way on their own. It is possible (even likely) that this first book only sets the stage for the full journey, and Nathaniel will find the good in life yet, but this book, on its own, is not something I can say I would recommend to other like-minded readers. (C. Orthodoxy has a great post about this that goes into much greater detail. But beware of spoilers.)
Overall, as an adult reader, I found this book compelling, and I think a child with a well-formed conscience could read it and take something away from it without his own sense of morality being affected. Still, I don't see this at all as a Harry Potter read-alike, and I would not feel comfortable giving it to readers under age 16.