The story is very coherent and easy to comprehend even though the pacing is as fast as a bullet train (It's a common trait of all works of Idea Factory). The writers managed to insert several unexpected plot twists and shocking revelations, and the most amazing part was the final scene. It's as deep as the Mariana Trench and the End of Evangelion combined, extremely thought-provoking and philosophical. The meaning of the final scene can't be comprehended without an insight into the inner world of the characters, because there's so much development and drama in these 18 minutes. Note that all the characters have names, however they can be seen only in the credits roll. Probably the authors wanted to show how universal their characters are, and that's not surprising considering that one of the main plot lines is about the eternal problem of unrequited love between people from different social groups.
Art is definitely the strongest point of the series. Not only the character designs are at the incredible level of typical eroge, Idea Factory decided to use modern technologies to the fullest. Unlike Mars of Destruction which had old-style graphics, Skelter Heaven offers us GLORIOUS 3D robots and space monsters. Even though the characters aren't 3D, they are inserted into the CG models, and it looks especially innovative. Even after five years, other studios still can't reach this revolutionary level of art quality.
Sound in Skelter Heaven isn't as awesome as in Mars of Destruction, there aren't any Beethoven's pieces in the soundtrack. However, the creators didn't hesitate to make an opening and an ending for a single episode OVA, and decided to hire top-class voice actors who did their job of conveying the emotions of the characters perfectly. I'll never forget the heart-rending scream of the heroine when she was in despair during the fight with the overwhelming enemy force.
Overall, this OVA is very enjoyable, it'll literally make you laugh and then move you to tears (though you'll probably laugh most of the time).
