This was not my favorite of the series, but given this is one of my favorite series, that is not really saying anything bad about it.
Spoilers for the previous books in the series will follow.
The introduction of Artemis and the mythology surrounding her was fascinating. She's always been one of my favorite goddesses, and it was nice to see her make an appearance and to have her virgin mythology handled in a way that made sense.
Bianca, Nico, and Zoe are great new characters, though some are certainly more likable than others. In the end, though, they all grew on me quite a bit. I was not expecting how Nico and Bianca's story played out, and it was intriguing to see it go that direction.
Having Annabeth out of the picture and in danger was upsetting, but a great drive for a quest. I really love Annabeth (and I think Percy is starting to realize he does, too...) Talia did not really replace her in the same way, but she is a pretty awesome character in her own right--very much larger-than-life (appropriate for the daughter of Zeus) and a bit difficult, but great fun to have around.
This is about the point where the series takes a turn for the darker, with the introduction of darker characters and mythology, as well as some death. It's not a bad turn, and it is handled extremely well, but be aware that this is dark in places. It's still extremely funny, and there are certainly the light-hearted moments that are such a major (and enjoyable) part of this series.
I enjoyed the ending, though I can guess it was probably divisive in the fandom at one point. I felt it was very appropriate for the character.
This whole series is seriously amazing and wonderful and I feel like everyone should read it.