I'll start off by saying that I didn't dislike Human Curiosity but I can't really say I liked it either. The idea is an interesting one but the execution is less than stellar. I'll also point out that I have only read the main story and none of the bonus chapters or the prequel.
One of my biggest problems with the fic was the writing style. Some may disagree with me, in fact a lot of people might, but I found the style quite dull and not very engaging. Now, it might just be me, but while I was reading there was nothing about the style that drew me into the fic and compelled me to finish and I kept looking for other things to do. This is what I mean about it being difficult to get through; when I'm more invested in a program about fishing than a fanfic of my favourite series, that is a big flaw with the story. Again, this is probably just me and my personal tastes.
Another problem I have with the fic is the way the characters were written. I remember thinking that this fic brought nothing new to the table in terms of writing the characters. I get that this is a fanfic and so there's only so much that can be done since these characters already have established personalities but I also remember thinking that the way they were written was very stale and had been done time and time again; not what I expected in a fic that came with a review praising some scenes as being "heartbreaking" which I didn't get from the way these characters were written.
The fic does have some good parts to it. Like I said before, the idea is excellent and I wish the author had done more with it, like looking at the impact of most of the world's nations vanishing and being tortured for a century. This could have also improved the characterisation as it would have provided a chance to see how the remaining nations cope with being the last of their kind but, alas, I'm only thinking of the story I wanted it to be rather than the one it is. The villain and his motivation is a little cardboard but the ending is satisfying.
Overall, the story is a decent one and, disregarding the issues I have with the way it's written, I'd recommend it. Just don't go in expecting something outstanding.