I have enjoyed most of Rick Riordan's Percy Jackson and Heroes of Olympus books.
However, this one had two major problems.
One, too many main characters. It was hard to keep up with who was doing what, and it seemed in an attempt to try to keep the story line clear there were always several characters milling around doing nothing. No specific storyline could get fleshed out as much as I would have liked because all of the characters needed time devoted to them.
Two, this books position in the series made it drag. We can tell that we are getting close to the climax for the series and are ramping up for the big fight at the end. However, that is all we did in this book. This book lacked a good strong storyline of it's own and read like it was just a really long trailer for the next book to come.