I've been looking into openings that I'm interested in starting out with the last days (still haven't started studying them really though, I will soon, If I can put My System aside for a few hours.), and I'm thinking about The Kings Indian Attack as white, but I have some questions about it.

What lines, as white, would I have to know except for the KIA to use it? I understand that there's different strategies depending on the opponent, but I'm fine with that, I'm just wondering, against what set-ups won't it work at all?

And how sound is the KIA really? I've read that at Master level the whole opening seems to be pretty much disbanded and it's rarely used, how far up would the KIA work?

As a fan of the KIA myself, I find my biggest challenge to 1 Nf3 is ... c5 which can be be wiser to change to the Reti opening. Also it can be wise to move into a transposed version of the english opening in certain situations.

As for it's viability, it depends on the situation. I love to use it in online games where my opponent has no idea what my skill level is. 1 Nf3 looks like a newbie move, and it can be fun to prove that it's actually a very viable first move and one which I am very familiar with.

The KIA was a favorite opening of Fischer's, although I think he would often transpose it by starting with 1. e4. Realizing I have no idea what your skill level is, I'd advise against working on hypermodern openings until you have a solid understanding of open, closed and semi-closed openings. Many people are drawn to KIA because of it's apparent simplicity, only to learn (or not) that it is based on advanced principles. To me it's like trying to learn how to through a curve ball before being able to hit the strike zone.

The reason that Grandmaster's don't play the KIA is because that it's too sound. By not taking any serious risk, or forcing anything, White often lets black develop a fine position. Grandmasters don't like to concede the advantage of the White pieces so easily, but you aren't a Grandmaster.