I teach 6th grade Social Studies in the South Bronx, and many of my kids love these books. It's easy to see why -- the storyline, characters, writing (especially the writing, which seems to be so poor in so many young adult books, even popular ones) -- are excellent.
However, Riordan does make one mistake that is an indication of American racism - he says that Western Civilization started with ancient Greece, but ancient Greece didn't exist in a vacuum or pop up out of nowhere. It was heavily influenced by ancient Egypt, but as a culture we don't like to acknowledge our African roots. I'm not saying that Riordan is racist, just misinformed about the origins of Western culture. Hopefully his next series will be about Egypt!
Additionally, this book has made me a better teacher because I am better able to empathize with my middle school boys who cannot sit still. Percy Jackson is that boy, and Riordan's explanation is lovely -- that his ADHD helps him in to survive in battle. Now, when Joel can't stop fidgeting and turning around in his chair, I picture him saving my life against mythological Greek monsters : )