The 8-year-old I babysit for just finished reading these books, though I was actually thinking about reading them myself long before he started them. They are written for a much younger audience than the YA books I prefer, but they were quite good, and with a sense of humor that seemed to help make it accessible to older readers as well.
For instance, pay attention to the chapter headings, as they are often quite sarcastic or amusing. For instance, the chapter in which Percy is confronted by the minotaur as his mom is trying to get him to safety is called "My Mom Teaches Me Bullfighting." Heh.
For those who haven't heard of these books, Percy Jackson finds out in The Lightning Thief that he is Poseidon's son. Since he is a powerful demigod, he has to go to a special school as soon as he starts getting stronger, because monsters from Greek mythology (the Furies, the Minotaur, etc.) keep trying to kill him. And before he can start to feel safe at his new school, he finds out that he has been framed for stealing Zeus's master bolt. He sets out to find the real thief, and to clear his name.
Besides the humor, it's cleverly written, and I can see why kids would love it. Percy is ADHD and dyslexic, but it turns out that it's because he's special — as his friend explains, his ADHD is because of his fighting instincts, and his dyslexia is because his brain is hard-wired to read Greek, not English. Genius — that makes Percy even more of a favorite hero for modern day kids.
It's a great adventure story with all of the elements of Greek mythology, too. Not just the monsters and the gods and demigods, but also the backstory and the trials that he goes through on his way to confront the real thief. I plan on reading the rest of these books as I find time — they are fast reads (from an adult's point of view) and plenty entertaining!