Definitely i didnt watch this because of the anime poster. 
On MAL forum, actually i made a thread about bad anime with awesome song, and this anime is one of them. Seeing it got average score, which is high for old anime, so i assume this anime surely have merit to watch.
Focusing the alien storyline, it began when advanced alien have breach in their ship that carrying some monster eggs. They crash landed to forest area near our protagonist house. She then came closer to discover advance spaceship as well as deadly monster going rampage. Unreluctant, she forced to transform a combat unit, merging with artificial lifeform. After the end of the battle, the lifeform somehow transform to become her clone, so they now live together while attending same classes. From here on, protagonist life changed for better so she can become more confident.
Lets just say, i dont care much about the fight aspect of this anime. It feel like a monster of the week, with subpar animation, no proper choregraph or even rewarding scene. The design of monster somehow kept evolving inbetween episodes, yet it still focused more on more tentacles for some reason. While the school life aspect is more interesting, it suffered from slow pacing that feel like im watching a slow burn emotional rollercoaster anime. Big problem is probably double runtime, so this is kinda like 26 episodes anime.
Definitely not average quality like i thought. Reviewer said positively on MAL, but personally it is just sloth to go through. Even though there are some emotional moment, but to me it feel like i just get more emotional damage.
Oh as a bonus, here is the intro of anime. Kinda weird right?