The book is a good one.
This sci-fi fantasy for middle grade kids is about an exciting and believable world, dark and scary enough to keep the pages turning. The characters, human and otherwise, are realistic and I cared what happened to them. I especially liked "Boots," the little sister who acted like many two-year-olds I've met but wasn't just there to be cute. Her behavior caused some important incidents in the plot.
Gregor the Overlander is the first in a series. Unlike some series books I've read, the plot comes to a satisfying conclusion but it still leaves possibilities open for future books. Kids who like excitement will probably enjoy reading it.