I wouldn't want to discuss this whole series in a critical way because I have always loved the Olympians universe ever since Percy Jackson. Doing it would be way too biased.
It took me about two years since I finished the Mark of Athena, the third book of The Heroes of Olympus series before I got my hands on a copy of House of Hades, its fourth installment. I'm not usually the type to wait for two years before reading the next book after an eventful cliffhanger but I guess school work has distracted me from getting a copy of the book.
But I ended my 3 year journey with the Heroes of Olympus in two weeks, when I finally found the time to grab my sister's copy of House of Hades. I plowed through the book in one week and then proceeded to Blood of Olympus a few hours after finishing the fourth book. I finished it in one week, because while my head was pounding with a headache thanks to colds, I wanted to know what was going to happen next sooooo badly.
I must say, Rick Riordan didn't forget to give his readers a pleasant, heartwarming conclusion to this Heroes of Olympians arc even after his ridiculously heart-breaking cliffhangers (I'm watching you, Mark of Athena.)
This might be late, but thank you Rick Riordan for creating such a wonderful universe. :)