It's a good read for a younger girl who wants a heavier meal than some fantasy books out there. The story is reasonable, though slightly cyclical in nature and lacking in psychological depth. I definitely recommend starting way back at Poison Study if you prefer your worldbuilding to have 100% context. If you want sufficient context on Opal, go back to Fire Study and start there.
It's a hefty read for a QP sized paperback, coming in at 488 pages. I definitely got my $14 worth out of it. It is not, however, a difficult read.