The original plot of the series is nothing too special. I mean, there are bad guys and people fighting against those bad guys. Obviously it is set in space so obviously there are nice and powerful mecha to fight against those bad guys. The new thing is that you can also fight them with songs. I was a bit surprised at that; at first I thought that Sheryl and Ranka were just regular idols, not that their songs were anything relevant to the plot. I was pretty pleased to learn that I had thought wrong. I still think it was a marketing technique for said songs but at the same time I thought it was well-integrated into the story so I did not mind as much.
On of the other side of the plot is the whole political aspect it had. It didn’t have much, but enough to keep me interested.
Last but not least, the romance. It was indeed frustrating at points, you can count on Alto to be oblivious and Ranka to be one of the most shyest girls ever. But in the end it all worked out. Even if that end did not seem like a closure at all, there were enough hints through the series to keep me satisfied.
As for the characters, I think I liked Sheryl the most. It is pretty rare, my preferences usually go to the villain. But I found Sheryl to be pretty strong-willed and strong in the more general sense of the term. She is not even the diva I thought she was. A nice, strong girl, and she is an idol. I love idols. 2D ones.
I disliked Ranka a bit more, as she is never sure of what she wants to do, never sure of anything, and not even sure that she likes to sing. It seems to me that the only thing she was sure of is that she loved Alto. It is hard to get attached to a character who only has that going for her.
As for Alto himself, I still do not know what to think of him. Sometimes he felt like an annoying bo-chan, sometimes he felt strong, sometimes even a bit sensible. The only thing I am sure of is that I love his seiyuu, Yuuichi Nakamura. (That already is something…)
To sum up my opinion of the anime, well, it has so many things that it is hard to get bored while watching. And this is a very good thing. I am recommending it!