Eragon got some stick because the plot so closely mirrored Star Wars and both of the previous books have taken their fair share of flack about the lack of originality.
I’ve enjoyed the series, though the books are a little childish, but Paolini was only young when he started writing them (and is still only 26) and the books have matured with him, tackling more complex issues with each one.
Likewise, there were only going to be three originally. It has expanded to include a fourth, upcoming, book, and so has changed from a trilogy to a ‘cycle.’
Brisingr is a fairly hefty book, but it’s a nice read, nothing too heavy of slow. One thing you have to admire is the scale of story. While the overarching plot has been known for a while, you didn’t get much scope of this vast conflict that has consumed the whole land, but Brisingr opens it up.
That’s not to say this is classic fiction, but while it might not stand next to the great fantasy epics, it has eclipsed most with it’s reach and that’s because the characters are engaging enough and it offers some home comforts (familiar races, etc) mixed with enough new things (for most of us) to keep it interesting.