Fun is a great word, isn't it.
Story- Mediocre is what I'd call this story. It makes no leaps, nor bounds. Chu2 simply repeats the same struggles we've seen before, but with a different setting. The action sequences are very good, but the rest aren't interesting.
Art- Nothing special, but nice during the action sequences
Sound- Nothing special.
Character- Rikka's character was outstanding. Her everything was awesome. Every scene without her was always lacking something important. She was the purpose of this show, and in turn, is the only character worth mentioning. Everyone else is either flat or uninteresting. The MC is shit, completely average and forgettable.
Enjoyment- My impressions of each scene ranged from "oh, that's cool" to "meh." Rikka being the only fun character doesn't help this.
Overall- I don't have many thoughts on this anime. It's forgettable because it doesn't stand out (even with it's cool premise). 6/10