I liked this one better than The Mark of Athena, though I still keep finding flaws in it—so no higher ranking from me here.
What I liked:
* Nico. I've always had a soft spot for the poor kid, who sure wasn't the strongest demigod in the Percy Jackson series itself, went through his lot of crap, and ended up so lost, confused and convinced he couldn't trust others, that he did everything alone... thus ending up even more isolated. But he's strong in his own way—I guess walking alone for so long does make you grow no matter what. In this book, we also get to see at last something that had been plaguing him, and I just found that great (even though I have my qualms about it, too; see below). You know what? I want THIS GUY, yeah, the son of Hades, the one with the permanent circles under the eyes and gloomy aura of doom around him, to get his love interest in the end. I know it'll never happen, but trust me, if I were into writing fanfiction instead of stories about my own characters and worlds, I'd do exactly that for him.
* Bob. (Now I need to read the Demigod Files, because I had absolutely no idea who he was, and wondered if I had missed a chapter somewhere in a previous book.) Also Damasen. I found it really interesting that those characters not being total evil guys happened in Tartarus, the very pit of despair, and a hellish setting to boot. It gets to show that the best can happen in the darkest moments. Anyway, how couldn't I like a Titan janitor with a broom-o'-doom and a half-skeleton calico cat? (Granted, I tend to like things that are over the top, at least from time to time. Especially in darker settings.)
* The darker setting. As much as I liked Percy's levity in the first series, the heroes are grown-up now, so it makes sense to see them confront darker events and monsters.
A relationship I didn't expect, and that was a nice break from the old Percabeth or Jasper (Pison?).
What I didn't like:
* Nico. Oh why, why couldn't we have chapters from his PoV from the beginning? Sure, it would've spoilt the bomb, but... but... I'm sure he'd have been more interesting than Jason. (Who, by the way, accepts the Big Reveal about him a little too easily. It's a nice lesson in tolerance, but it feels exactly like that, like a lesson, and not as a 100% human reaction.)
* Jason is still his bland self. I've never been able to relate to him, and I still can't here. So, all right, the guy's got trouble conciling his Roman training with what he lived with the Greek demigods, and doesn't know where he belongs anymore... but it's just not working for me.
* It's the same old formula, and even though I liked it in the beginning, I'd also appreciate seeing something else at some point. Also,
* The enemies are as dumb as ever, and always get tricked the same way. I suppose it's the only way, considering brute strength would never work against giants and immortal monsters, but... OK, Nyx. Come on? Nyx. *facepalm*