In what was originally touted as the final book in the trilogy, Brisingr neither disappoints nor impresses and serves primarily as a place-holder in the series. There are some significant plot moments and a plethora of new characters, but all-in-all the novel serves only to highlight the atrocities the Empire has committed and is willing to commit to remain in power.
It is in Brisingr more so than any of the other novels that readers see what Paolini is trying to do (whether he is successful or not I will leave to each reader). He has created a world and he now has to fill it. Not only is he writing the story of what is happening in the world, but he is attempting to enumerate the myths, legends, and histories of the various inhabitants of Alagaësia. Whereas in Eldest we learn of the elves, in Brisingr we learn of the Dwarves and Urgals (bipedal creatures with huge horns growing out of their heads, think minotaur, but less bull like).