Summary: interesting and captivating for the first 10 episodes then digresses into a convoluted mess.
The anime is very good. The voice acting is excellent as is the directing. The writing is anime-acceptable. It's understood that when one watches anime, it's likely to have anime qualities. That includes cliche, standard anime tropes, and a bit of goofy. That's part of its charm.
The story starts off unique, slowly building on the characters and story line. At the beginning the lead character (a whiney teenage girl) comes off pretty much as a whiney teenage girl. There is growth in the character, but not in any way that is truly important to the story. The other characters are more interesting, but about half of them unnecessary to the plot.
It's a "giant robot" story so one can accept typical giant robot plenty-of-action. Unfortunately the story line although interesting, remains rather simplistic. Just as one is expecting everything to be explained and sewed up, they pull a typical "What is going on now?" deeply-emo "plot twist" too often seen in anime, and the story just plops into a disappointingly unoriginal end mess. Two of the characters become so confusing in nature, the viewer loses track of who is intended to be the good guy / bad guy, or if both are both.
It is worth watching, but only just. Parts of it are very enjoyable, other parts not so much. A better and more logical ending would have gone a long way in making this a more enjoyable overall animation. As it is, it runs along for a good sprint but then trips at the finish line. In the end I found myself thinking, "I'm glad it was tied up in 12 episodes"... and wishing it had more thought put into the final two.
Extra note: if you enjoy English lyrics being sung in heavy Asian accent, the theme song will be a real prize. ;D