Why the ending ruins it...
I see it was trying to go with the whole "omg... we have no hope now!" thing like many other films have tried to do before. But this one got it completely wrong!
Why?
Oh because I dunno... The people on Earth aren't retarded and the girl wouldn't just let it slide and not tell anyone about what happened. It's just completely unbelievable - yes I know a lot of the movie is unbelievable, but I'm not talking about the rest of the film and to me the ending was meant to make you feel creeped out and all this...
Sort of like the ending of... Invasion of the Body Snatchers or something where everyone gets taken over in the end and there's no hope for anyone. Whereas this film tries to convey that by saying "well, this cyborg guy impregnated a soldier on a planet thought to be unpopulated, now they are returning to Earth and nobody is going to notice, ask questions and the girl isn't going to talk to anyone about any of it.
The only reason that this ending could work is if the writers of a sequel decided to write her character like a rape victim. "oh he's gonna kill me, so i can't tell anyone about this baby or the father". OR they write her as an anti-abortion kind of person who just can't get rid of what's in her whomb wether it's a giant fistful of dograbbits or a cyborg killing machine.
Anyway, the movie was alright up until that ending.
Perhaps it was just her having a nightmare. But we all know that's the shortcut out of a poorly written script ;)