My 7 yr old son and I have been enjoying this series for a few months. Some times we read it together, but more often than not we sneak off and both read ahead. (He is quite the advanced and voracious reader for 7!) We've gotten so into them we moved onto Heroes of Olympus and are now reading the Kane Chronicles while anxiously awaiting the next HoO book.
In an effort to test my son's reading comprehension and the overall educational value I looked up a few Greek Mythology quizzes. I actually had to use the middle school ones, as they don't learn this material in elementary. He knew all of the Greek Gods names, roles, lore, interrelationships and thanks to Heroes of Olympus, their Roman equivalents. He also did pretty well on the Greek heroes (Hercules, Perseus, etc.) and didn't miss a single monster/beast. (Of course!) We got into the series to get some extra mommy/son time but I love that he's picked up this subject matter and truly understands how all of the pieces fit together. Before reading this series, I'm not sure I could tell you who was married to who, whose children were whose, etc but now both my seven year old both understand the complex interrelationships of all of the Olympian Gods and Goddesses.
The book is an adventure series and they do spend a lot of time fighting monsters. Demi-gods do get injured but fortunately they always carry a magical food that cures their injuries. It does subtlety address more mature content, as other reviewers mentioned, but in a way that a seven year old doesn't even raise an eyebrow. (For example, one camper will tell another camper "Zeus is my dad." He will not say "Zeus is my dad. He had an elicit affair with my mom in the 90s, left us and went back to his goddess wife, Hera, who now hates us." It's really the more mature reader who is reading between the lines in those situations.)
There is next to no consumerism. There isn't a toy line or Lego series (much to my son's disappointment) and very few collectors/fan items out there. They occasionally reference Diet Coke, but there really isn't any branding or merchandizing. Fortunately, we've been recreating scenes and weapons with legos, cardboard and lots of gold duck tape!
If you are continuing with the series, it is important to note it is much like Harry Potter. The children start off as pre-teens and then grow up. So by the the Heroes of Olympus books, the girls are starting to take more of an interest in the boys and are sometimes dating but relationships aren't really a factor in the Percy Jackson series, which comes chronologically first.
Another point of note - the books and the movie are not at all aligned. Plot, character appearances and even geographic locations and quest purposes are changed. I didn't like the movies and think they are a poor representation of the series. I am hoping Disney will consider a reboot in the future.