It's a cute series, the ideas are well thought out and the characters are memorable. The sequel series has it's own set of charms but in terms of adventure, the PJO series takes the cake. I've finished it through and through, but my favorite is Battle of the Labyrinth.
I really like the series and it is very humorous. I enjoy the plots and learning about Greek and Roman in the following sequel a lot. I just finished the House of Hades.