All the otaku-oriented fanservices rolled into one.
Unless you're looking for 13 episodes of super-powered teenage schoolgirls, mini-harem, meganeko, lolis, heaps of fanservice, and a +1 bonus valentine episode that didn't think the above was enough then I suggest not to read any further.
On to the review:
It's just a bunch of schoolgirls fighting off an alien invasion composed of loli girls. If the premise isn't childish then I don't know what is. With this you can expect the monster of the week and all those things above. I can't really expound anymore since there's nothing to expound about.
The art, sound, and character designs are the same as Hand Maid May so I think you know what to expect. Poorly designed male cast with cute girls. There is the constant proportion problem that will made me wonder whether the animators were stoned during the conception of this anime.
Amidst my poor rating of this title, it was enjoyable in a guilty pleasure sort of way. It's not something I can recommend but if you want to experience what it's like to have all the otaku fetishes rolled into one anime then you might just enjoy G-On Riders. Then again maybe not.
Enjoy the anime but make sure that the "ordinary civilians" aren't around when you watch this not so secret shame of anime.