Oh my, this really wasn't a satisfying concluision (not that I was expecting one). The ending was just too damn convenient and perfect (just like the ending of "The Heroes of Olympus") - it really didn't work for me. It probably doesn't help that I didn't give two shits about any of the characters throughout (except for Nico but alas!). 
 I can see how children can devour this series, and get a lot out of it, and that is great. Personally, I think I was a little too old to really truly enjoy it. It was too repetitive, too badly written and just not memorable enough. But to end on a good note, I cannot stress enough how high my appreciation is for his choice of interweaving some Greek myths into his narrative. I love that idea so much, even though it is ruggedly executed...