Definitely not the best one in the series so far. I've even considered giving it a 1-star rating, but it's also not the book I've disliked the most either. I guess it was just "meh" for me.
Although the "Percy Jackson" and "Heroes of the Olympus" series have their flaws, I still enjoy them, as the kind of light reading I fancy when I really just want something fun and easy to read. Not that all is fun and roses for the characters, but I liked Percy's tone in the first series. Though not witty, it had snark, and I'm always up for snark. The switch to third person in this second series, though, has been a little nerves-grating as far as I'm concerned. I noticed it in the first book, then thought "nah, I must've been in a bad day when I started reading it, is all", then the feeling came back. With book #3, the feeling has stayed.
The setting is still highly enjoyable, all the more because Rick Riordan takes great care in using myths that aren't so well-known. Unless you're already a specialist in Green and Roman mythologies, you're very likely to find at least a few gods, goddesses, heroes or monsters you didn't know about, and this is just great.
However, I think the main problem for me with "The Mark of Athena" was the different points of views. I usually don't care if there's one, two or ten: I can follow a story no matter how many POVs it contains... provided they sound different enough. Here, I thought all the characters had mostly the same "voice". Percy's lost his sass, Piper's annoying, Annabeth doesn't sound as strong as she was in the previous books, and Leo... well, OK, I like Leo, though others will probably find him annoying, too. Jason, Frank and Hazel don't get to have a voice, and it's too bad. I liked Frank in book #2, and Jason definitely deserves more development, because for the moment, he's still more a cardboard cut than a real person. Also, I have a feeling that the characters are a little too much mirrors of each other, so what worked in books #1 and #2 doesn't work so well anymore when the seven are gathered. (The original Percy/Annabeth/Grover trio echoing Jason/Piper/Leo, and Percy/Hazel/Frank following the "1 girl, 2 boys" trio pattern in "The Son of Neptune", only with Percy not being the love interest this time.) Last but not least... Personal preference, sure, but why couldn't we get Nico and Thalia instead? I don't resent the "new" characters, but I'm convinced some of the "old" ones still had so much more to "say". I'd have preferred a, let's say, Nico/Thalia/Clarisse adventure to the Seven's angst- and mush-filled trip to Rome.
I don't even want to talk about the enemies. Please, please, give us clever villains. The giants were just... so dumb.
...And yet, I still want to know what's going to happen in book #4, in spite of the annoying characters and diminished writing style. Somehow, this series keep calling me back. This is mostly weird.