I have been following this series since "The Lightning Thief". I have loved every book and have grown close to the seven main characters and even the two that became so important in this last book, Nico and Reyna. I think Rick Riordan did an amazing job at fleshing out each one of them and making them grow with the series. However I felt that this last book could have been combined with the fourth book and it would have been a more powerful story. 
 The group has been plotting on a way to defeat the Gaea before she comes back to Earth and kills everyone and the whole series has told us about how impossibly hard to beat she would be. While the threats they faced felt real, the way they handled their mighty opponents has always been questionable to me, they're opponents are supposed to be incredibly powerful, but they are always defeated in a such a simple way. The book does a lot of this as it drags out the final battle for 30 or so chapters. Defeating Gaea is supposed to be impossible by the end. I won't go into how it was handled, but it feels like Rick wrote himself in into a hole. He got out of it the only way he could have. I wasn't dissatisfied with the ending. I think it was appropriate for the core audience of the book. I couldn't tell you how I would have brought such an epic adventure to an end, but I was sad that it had to come. I would recommend this series to anyone.