I will try to keep this as spoiler-free as possible, but beware regardless.
There was nothing wrong with the art/sound quality, they were the same as in Index. However, the storylines were much less enjoyable and...Kuroko was prevalent.
Kuroko's main purpose seems to be just the moe loli lesbian. Her shrill voice coupled with her annoying actions make her an extremely bad character. She's not as annoying when she's being useful, but as most of the scenes in Failgun seem to be just the characters messing around, the bad highly outweighs the good.
Since I could go on for pages about how much I hate Kuroko, the only reason I didn't rate this a 1 in character is because she's the only horrible one. Even though Mikoto is a tsundere (the bane of my existence), she's actually not too bad, at least compared to other characters I have seen (Taiga, Louise) from other anime (Toradora, Zero no Tsukaima). Uiharu is an airhead (which do annoy me from time to time) and I actually grew to like Saten after a while.
The story arcs were awful. At first, it was trying to be cute and funny, but after a while it started to get "serious." I put it in quotes because it was all extremely lame and unbelievable. And here is where the minor spoiler comes in. The end of the first "serious" arc completely spat in the face of science by playing binaural beats through speakers, when they actually require the sound to be played through stereo headphones to the recipient in order to work at all. And this is what resolved everything. And this really should stay true to some kind of science, this anime even has science in the title!
In conclusion, if you truly appreciate Index and science, stay very far away from this side story. It's not worth watching even one episode.
