I found Eragon to be a good fun read. Not a great book, in my opinion, but good nonetheless. My two biggest complaints about the book:
1. I thought it was highly derivative. But a 9-year-old who hasn't spent a lifetime reading Tolkien and other fantasy authors as well as playing tons of Nintendo and Sega RPGs shouldn't have problems with this. I've read a lot of fantasy. And as I was reading the book it was fairly apparent to me what Paolini's influences were while writing.
2. The author was 15 when he wrote the book, and I felt he was often using big words for the sake of using big words. A 9-year-old might need to hit the dictionary to look up words like "scintillating," for example.
Having said all that, I'd definitley recommend this book for a 9-year-old who's read the Harry Potter books, the Earthsea trilogy, Madeleine L'engle and the other usual suspects.
I'd also recommend The Thief Lord and Inkheart by Cornelia Funke. Her book Dragon Rider can be summed up as Harry Potter meets Eragon.