I discovered Percy Jackson and the Olympians through Bookshelves of Doom last fall. It really seemed like a great series. First, despite the on-going rereadings of Harry Potter, there is still a void. While PJ doesn't have the fan-base or cult-following of HP, it's good clean mythological fun.
Quick Summary: Percy Jackson finds out he's a demigod, a son of Poseidon, except Poseidon isn't supposed to have any more sons because one of the children of the Big 3 (Zeus, Poseidon, and Hades) will have to decide whether to save or destroy Olympus on his (or her) 16th birthday. The ramp-up to this event basically makes up the story arc of the 5 book series. The Last Olympian is the last book in the series, though the blurb at the end makes it sound like there could be more--maybe in the form of a spin-off of some other demigod. (That'd be cool, but I'd worry he'd sound too much like Percy.)
The series is written in first person from the POV of Percy so you get a lot of sarcastic, teenage-y language. Nothing bad, but if you don't like the way "kids these days" talk, you might get annoyed. I personally find it amusing. I find high schoolers endlessly amusing anyway.
Let's see. I'm not usually a big worrier about spoilers, but it's the last book of a series and that's a whole lot of spoiling. I guess, I really appreciated the family ties, stick together theme that runs particularly strong through this book. So much of fantasy has this strong protagonist with superpowers who runs the show and I love it when authors bring in the little people--and make them big people. It happens with Neville in HP7, it happens here as well. "We do it with cooperation" (Psalty the Singing Songbook).
Why read PJ? Well if you like Greek myths, you like the series. It's fun to see how Rick Riordan modernizes the mythology and makes connections. I learned quite a bit, but like most books that concern things I'm interested in, it compels me to check out the real myths. (Sadly I don't know that I have a book about that. It would be great for the book challenge.) However, when my husband, the classicist (though he is not a big mythology person--he likes the non-fiction), misspoke about Artemis, I was able to correct him because I had just read about Percy joining up with the Hunters of Artemis. Fiction strikes again! So it's good. People could probably be offended by its neo-pagan focus, but it's a modernization of Greek mythology which was a religion, some of that is just necessary to remain true to canon. Otherwise, the other common HP arguments--defying authority and stuff--doesn't occur here. Percy is a good guy who tries to be honest and do the right thing. Also, this is a great book for ADD/ADHD kids. One of the premises is that Percy doesn't do well in school because his demigod brain makes him easily distracted (quick battle reflexes) and makes him dyslexic (the better to read Ancient Greek). Kind of cool.
Final note... Rick Riordan's written some adult fiction. I read one of the books from his Tres Nevarre series. It was okay, but not as compelling. Classic thriller/mystery novel (when I was burned out on Classic thriller/mystery novels).