I don't generally care for Mecha, but this was outstanding.
You follow the mistakenly united characters in their lives and sympathize with them inside the behemoth. The psychological plot and unique twists create interest value. Sacrifice and courage are learned, while maturity is gained for the good of the people. The grim game of life and death is only half of the battle.
Expect to fall in love with the opening song, as well as cry. Allow yourself to grow attached to the characters in such a limited episode amount.