I just finished Fire, the prequel-ish companion to Graceling. Although a little slow and, I thought, slightly confusing at the start, Fire will not disappoint readers of Graceling. The plot is driving and exciting, and thoroughly unpredictable. Like Katsa, Fire is a complex character who, during the course of the novel, learns about herself and others' perceptions of her in an interesting way.
Cashore's writing style has developed beautifully--not that it was bad before, but it has definitely matured.
I really, really liked Fire, and it kept me reading even after sitting there for several hours. Yet on my Goodreads account (friend me if you want), I only rated it 4 of 5 stars. Why? I think because Graceling resonated with me much more deeply than Fire did. Even Katsa's romance was a lot more to my taste than Fire's.
Fire comes with my high recommendations.