It took considerably longer than I expected (the World Cup didn't help, obviously), but I did finish Riordan's latest, The House of Hades, several days ago. I think I'm just getting a bit tired of the format; it wasn't bad, but obviously didn't hold my attention as well as his earlier books did.
In sections, it was very good, but didn't hold together as well as many of his earlier books.
Part of that is my fault, of course; I have far more distractions, these days. But I think part of it is just getting a bit tired of the format. Travel a ways, part of the group splits off for a small-group quest, they succeed, repeat.
And the other part is that it's a larger group, so you don't get nearly as invested in any group. There's some pluses to that, in that you can get different variations via different combinations, but it doesn't balance out for me.
And several of the quests just seemed too easy to me. Or, at least, the difficulty was only shown externally, maybe. I'm having a hard time explaining it, I think.
As far as continuing the storyline, it does a good job. The heroes soldier on for the book, and find some unexpected allies along the way.
Frank got two big scenes, neither of which did much for me. The first involved killing a bunch of monsters that weren't threatening when they were left alone. So it didn't seem especially heroic to me. The second was a little better, but... It was along the lines of leading an army which, while important, is not as personally involving. Perhaps, also, that part was explained in less detail (not enough of movements, and why each one was important, maybe).
Anyway, I'll still get the last book (I'm pretty sure there'll only be one more), but a lot of the excitement and enjoyment was lacking. Perhaps if I re-read the original Jason and the Argonauts (which I haven't read since middle school), that would help. Though my suspicion is that this series isn't nearly as much a retelling of Jason and the Argonauts as the Percy Jackson series was a retelling of the Labors of Hercules.