I really like this series (have read the first two so far), and it's a shame that it isn't more well known.
Riordan has done a very clever thing - he's entered the cluttered young adult fantasy arena with plots woven from the old Greek legends. So he has actually claimed the stories that have since inspired Rowling and others. I mean, the parallels are almost funny - in this series we have a prophecy about a young savior (hello harry potter & golden compass), and we have a dark lord (Kronos) slowly rebuilding his strength as he prepares to take over the world (hello voldemort!!).
And though you might expect the greek tie in to lead to overly-dramatic writing, the book is actually really fresh and funny. The God Poseidon for example is a Jimmy Buffet / beach bum playboy figure.
I highly recommend these books. They're more quickly paced than Rowling and Pullman's books, more readable (because Riordan isn't building up detailed backstories the way those authors do), and quite simply they're great page-turners.