I think this is the general feeling among children's fantasy people--that those who don't know fantasy are the ones who rave about the books that hit it big and are plain and derivative (e.g., Eragon). Inkheart isn't derivative so much as too much in love with books to have a real plot.
I found that once I’d put the book down, I had little to no desire to pick it back up again. I just didn’t care enough to find out what happened next, and I was never really worried that things wouldn’t work out in the end.
This is exactly what happened to me. I loved
The Thief Lord, minus the truly annoying fact that it really shouldn't be fantasy, because the only fantasy in it was the deus ex machina ending, but I bought
Inkheart thinking it'd be better, and because all my classmates and teachers were raving about it. I think I got up to about chapter 5, and never went back to it.
However, Cornelia Funke herself is a very engaging and knowledgable speaker. She really knows children's books and how to teach others about writing--she came to my school and I really enjoyed hearing her speak. So, perhaps part of the raving is because people who meet her like her so much.