I find it increasingly hard to give a star rating to books. With this one, I tried to rate it as I would have when I was 30 years younger. I think it's a great children's adventure, but unlike a lot of readers, I didn't feel this particular book crosses over to an adult audience (i.e. the me now, not the 30 years younger me) as well as some other books meant for the same age group do.
That said, Collins - as anyone who has read The Hunger Games probably knows - is a strong writer, and it shows in this, which I think was her debut novel, as well. She's created an interesting world, and her characters are strong enough and easily distinguishable from one another. I even liked the way she had the different groups inhabiting the Underland speak, so clearly separated from modern American English.
I think it was just a little bit too "young" for me - I like children's books, but some of them are more of a joy to read as an adult than others. That said, I might some day read on in this series, but I'd probably need to be in the right state of mind for that.