This book is definitely as good as any of Riordan's other works. He very skillfully gives each of the seven characters their own portion(s) of the story told in their point of view without losing the forward momentum of the plot. Each of them has some task to accomplish in order for their common goal to be met, and I honestly felt interested in what was happening to all of them.
There is also some great character development and darker moments. I particularly like the growth that Leo undergoes and the attention he gets since his is a character that could merely provide comic relief and mechanical skill, but his growth is more realistic, though in a conventional way. The revelations about Nico's struggles were annoying at first, because I usually feel (view spoiler at Goodreads)[inclusion of a gay character in a mainstream novel is an intrusion of the author's compulsion to be PC. (hide spoiler)] I don't know if Riordan always intended for Nico's storyline to go in that direction, but the treatment seems incredibly authentic to the feelings and reaction a boy in his situation might have, so kudos to Riordan for managing to engage my sympathy despite my initial reaction. Frank's growth, pardon the pun, is fairly predictable, but I enjoyed Hazel coming into more of her own and I look forward to her mastering her abilities even more in the next book.
As for Percy and Annabeth, I agree with some of the criticism I've read that, as awful as Tartarus is portrayed, the lighter/humorous moments are maybe not realistic, but it is a MG series so they are still appropriate. Seeing these two stretched to their limits and examining the morality of their past actions during other quests deepens this story beyond just an action/adventure fantasy. I really like how Percy is forced to reflect on his treatment of Bob. We already know Percy is not perfect, but this kind of reflection and growth adds another layer of depth that makes him more admirable.
Very satisfying read, highly recommended!