Finally, I finished reading the series. I attempted to read the series two years ago but I wasn't able to read it through and through. All thanks to maternity leaves and breastfeeding, I have ample time to read books.
The series was meant for children. It's easy to read and the story is light. Unlike with Harry Potter (sorry for comparing), the tone of the story didn't mature every after book. The theme usually starts with a quest then fighting monsters along the way, on to fulfilling the prophecies for each quest.
The characters were easy to remember, maybe because Greek mythology is familiar to us. I haven't checked, though, if it followed the actual myths.
In fairness, the last book "The Last Olympian" was just based on the battle between Kronos' army and the demigods. It was full of action and drama but still retained its light tone. The deaths and losses were masked or weren't given emphasis. I guess, the author had it in mind the audience the book was intended for.
This series is fit for first-time novel readers. Story-wise, I'm giving it a 3 out of 5. Sorry but I have Harry Potter in comparison to this book.
P.S. Thanks, Sir Ronald, for loaning me your Percy Jackson series.