It is like wine, as it matures it gets better.
I was also frustrated by the end, initially.
BUT as days go by and I reflect on the different themes, story arcs and parallelisms between PON and Aspect-Emperor, the more I like it.
I mean Kellhus gets salted. Throughout the entire PON I wanted Kellhus to get defeated, because I was annoyed at how good he was at everything.
Then during the Aspect-Emperor series, slowly but surely as the full depravity of the Consult became clear, I started to support Kellhus, even up to the point where he became an Avatar of Satan (pretty much!), despite having no idea what was his plan. Just the one liner, when he gazed into the Inverse Fire produced so much awesomeness: Where you fall as fodder, I descend as hunger -- which put into full perspective the dialogues in the Great Ordeal about fodder and harvest, I presume it was Kellhus debating / discussing / making a pact with Ajoklis. Awesome stuff.
And now having changed my mind about Kellhus, he is salted! What a roller coaster of emotions!
Achamian is still alive, and I sincerely hope that he will be instrumental in defeating the No-God, so all is good.
There is design in the density of the series and all is forgiven.
It is a rewarding experience but only once you let go the notion of a clear end.