This was an action-packed, fun-filled adventure that is as good as Rick Riordan's other books. All of the characters are likeable (even the "bad guys"), fun, and completely relevant to the story. The plot moves quickly and the hints at the clues are fictional, but used beautifully. I've heard it described as a children's version of National Treasure, but it's so much better, especially since Nicolas Cage is nowhere to be found.
Amy and Dan Cahill, the main characters, are just normal kids that are set into a high stakes treasure hunt. They're not rich, strong, powerful, famous, or conniving like their Cahill cousins who are their competitors in the hunt for the 39 clues. You can almost believe their situation, their behavior, and all of their losses in life. They are fantastic protagonists, and I really enjoy them as the center of focus.
All in all, this was a great start to the series, and I have already breezed through the second before even getting the opportunity to write a review.