I'm surprised there hasn't been more discussion on this series (unless I've missed a thread). I just finished the 5th and final book today, and it left me wanting more.
The Percy Jackson books were very well though-out, with the stories intertwining and really coming together in the end. The author definately knows his Greek mythology, and manages to stick to the myths despite putting them in a modern setting. I've found myself doing some research in non-fiction books on Greek myths just to learn more about some of the characters and events that are referenced.
I'm sorry to see this series end, since I've really enjoyed reading about Percy's adventures.