With two comments questioning the quality of this series and a current average review score of 6.67, I assumed this would be complete garbage. Imagine my surprise to find that it is actually quite good.
The basic plot doesn't win any awards for originality: the machine that controls the planet's climate and robot forces has gone out of control and has initiated a program to destroy the planet. The one person who can stop it is Carol, a human-like entity created with control instructions embedded into her that can restore the machine to normal operation. While it isn't super original, it still has a few neat twists, and it is executed quite well.
The characters are surprisingly great, even the side ones. Both lead characters love each other (as is par for the course), but their love is so complete and unconditional that they're really more pansexual than lesbian. Carol can transform her body at will, so there's a bit of genderbending and the threat of sex between a male and female. For any yuri fans who are tempted to run away, don't. You can count the number of panels where Carol is male on your fingers alone, and she ends up not being in her male form for any of the sex scenes despite what was foreshadowed. In fact, the genderbending presence is so light that this series almost doesn't deserve the tag at all.
One thing to admire is that unlike your typical fantasy or sci-fi yuri manga that allows explicit sex scenes, the author of this manga didn't shoehorn a sex scene into every chapter (though he does show off bare breasts very often) in order to capture the attention of the audience (usually this backfires for me and I find it insulting). They only exist in one or two chapters, when it makes sense for the sake of the story.
I'd like to bring up the art last. It was also great. I have a hard time following the action scenes in manga sometimes, but it was pretty obvious here what was happening.
Basically, if you like sci-fi or yuri, give this series a shot. My guess is that you'll be surprised and it'll be better than you thought. It's not a masterpiece or anything, but I enjoyed it and would give it a solid 8.5/10 personally.