I was actually kind of ambivalent towards Akka during my first read of PON, and even on my re-read (followed by catching up with the other two released books at the time, TJE and TWLW). I never thought he was a badly written character or anything, but I suppose I felt similarly to what TaoHorror said above. To some extent I still do feel that way -- my favorite characters are definitely the more "take charge" types, or at least "take action" (which is often, but not always, literally taking part in an action sequence).
However, after TGO and the re-reads that followed, and especially after TUC, I can say that my appreciation of the character has grown considerably. I think the reason for this is two-fold: firstly, Akka benefits from re-reads more than perhaps any other character in the series. His wealth of knowledge and general musings (philosophical or otherwise) had a tendency to just sort of wash over me during early read-throughs, but after returning to the text continuously and not having to focus on plot elements as much, the various instances of Akka's self-reflection, his perspective on the other characters, and all of the little side-notes regarding the worldbuilding and such are not only easier to digest, they also become more interesting the more you unravel the mysteries and concepts at play in the series.
It's especially interesting to take Akka's overrall opinion of himself and his place in the world during PON (in which he is basically a very, very heavily armed conspiracy theorist), and then contrast that with the old, crazy Wizard of TAE. In many ways Akka changed the least of all the original cast from the first book onward, but not in a negative way -- it actually makes sense given the entire premise of the Mandate and Seswatha and so on.
In terms of sheer writing, though, I would confidently argue that Achamian and Mimara are wholesale the best-realized characters from across the series (with probably Cnaiur and Kellhus coming in right on their heels). Akka and Mimara's banter, mannerisms, and interactions feel increasingly genuine as TAE progresses, and by TGO/TUC it feels like the author is effortlessly channeling these two very complex human beings.
It's funny as I think of it, but Achamian might be the main character I'm most looking forward to in the next series, if only because he will finally be pretty much the only authority people have to turn to regarding the Apocalypse that is very clearly taking place across all of Earwa. In a way it's like only now can Achamian truly become Seswatha, especially since he has some very personal reasons to hate the Consult. Almost as if everything so far has been in preparation for this section of the story.
