tl;dr So, I'm going to review all 5 books in one review, because I don't have enough to say about each individually. Overall I enjoyed the series, but its not one I'd re-read over and over again. I'll definitely pass it along to my cousins and nephews and niece when they're a little older though for sure. (They're 5 at the oldest right now).
Description Percy Jackson is not your ordinary kid. He suffers from dyslexia, ADHD, and weird events happening around him that end up getting him kicked out of school every year. Finally, one day when he's 11, he learns the truth: he's a demigod and the son of Poseidon, and he's going to a camp for other half bloods to learn to train and fight against ancient monsters. Over the next 5 years, Percy struggles and fights against ancient monsters, trying to get the gods to believe that their toughest enemy is making his way back and is going to attack. With the help of his best friends Annabeth, Grover, and eventually his brother Carson - Percy takes on the evil beasties and is ready to fight to save Western civilization as we know it.
So there were a few things that overall bugged me about this series. Sometimes the kids didn't seem like 11 and 12 year olds, but then - and this pains me to say it - I guess I've forgotten what its like to be 11 and 12. My nephews are 5, my cousins are no older than 9, so its not something I'm used to anymore. I'm assured by my amazing child librarian friend fawkesylibrarian that they are quite true to 11 and 12 year olds, so I am taking her word for it.
I also wonder what it is about our leading kids dudes being black haired and green eyed. Harry, Percy, and I'm sure many others. It seems like a prerequisite these days. Which is fine with me, I'm fond of the coloring combination.
I really liked Annabeth's character. She's really very Hermione-esque. And I'm sorry to be comparing them to Harry Potter characters - I've been listening to those books again on audiobook so they're fresh in my mind. But seriously though, Annabeth is the daughter of Athena so she's got the intelligent bookish thing down. And I dig it. I love smart female sidekicks. I almost felt like this story should've been Annabeth's (just like HP should've been Hermione's because without her, harry would be dead by age 11). But Percy wasn't completely useless, or completely lost without her so that was good.
Of these stories, the last was my favorite. The first was good, but in all honesty i didn't get swept up into the story until the last book. The Battle of the Labyrinth was pretty awesome, but I have a soft spot for mazes with crazy battling tasks you have to go through (goblet of fire was my favorite). The writing style was definitely friendly for kids, and I think had I read them when I was younger, I probably would've liked them more than I currently do. But I did think they were fun and had some awesome characters, and I liked the use of Greek mythology as well.