I would guess that Riordan had an idea for a book and discovered the mass appeal of the Harry Potter series. He took his idea and ran with it recognizing that a market existed. Trying to not sound like Harry Potter, there are moments when he set himself up as too obviously different. Instead of fleeing his family during the school year, Percy Jackson flees during the summer. Instead of no parents and evil extended family, Percy has the perfect sweet mother. Those moments don't take away from the book, but they did catch my attention.
Percy Jackson has ADHD and dyslexia - appropriate quirks for today's middle school reader. I like that Riordan describes those characteristics as signs of half-blood relationships to the Greek Gods. It's novel and a fun take on an old issue. Oh yeah, and if you haven't read the book - the main premise is that Percy Jackson is half Greek God.
Which brings up the main plot of the book. Riordan introduces his readers to a whole slew of classic Greek mythology: Zeus, Hera, Apollo, Aphrodite, Medusa. Although these are less relevant in today's world, I think its noble to introduce this world to kids. Knowing who Hermes is makes the FTD symbol at florist shops suddenly make sense. Riordan takes the classic tales and adapts them to the modern world. It's fun.
As far as appropriateness of this series for the kids themselves? My son's only six and he's too twitchy to listen to a story this long yet. But, I think it is a great series for boys - a group who has been ignored as readers until recently. I've been told that the further in the series the more death and destruction that occurs so parents may want to pre-read depending on their child's interests.
For me, I'm anxiously awaiting for the rest of the series to show up in my kids Scholastic flyers.