The main issue is that it can be divided in two very distinct parts, and even if both of them are strong the turn it takes really hurts both. The first part is a nice classroom sort-of-yuri comedy/drama. A regular girl falls in friendship with the weirdo who sees people as robots (not in an utilitarian sense, completely literal to the booster pack and exchangeable parts). People stand in their way, problems arise, some drama, they build gunpla and eventually manage to become dear friends and even connect with other people.
The second half, though, is full blown sci-fi of the best kind. A small element is incorporated in a regular setting and it's slowly expanded until the point it breaks the universe. The escalation this second half has is incredible and it deserves to be seen just for how far it goes. I didn't exaggerate when I said it breaks the universe. It's an author exploring a premise to every possible degree, something that you can see many others completely failing to do while getting praised just for the initial concept.
The problem is that both parts don't really need the other. When I finished it I checked a couple of one-shot stories set in the initial part and I had already forgotten that this had been a happy school days gag manga. It took me to write this review to remember the drama from the past that closes that part of the story. I could only remember the last chapters, and when I first saw a recommendation for this was through a page of nearly the end. It was basically luck that I had forgotten why I was reading it and stuck for the girls in highschool beginning. I assume this comes from the original LNs and I really like the idea of the author first making a funny high school story just to hit the readers with a completely different sequel. But when you read the manga you don't have that clear distinction.
The art isn't too good although the gunpla look great and in the second part it's up to par to make the reader understand the story. Over all it feels like a subpar mangaka adapting a cult classic.
In many elements it resembles Madoka, it surprised me that the LN had come a year before that anime. They were probably responding to a cultural desire of almost-yuri + expansive concepts that affects the universe. Just like you have half a dozen Harry Potters in the couple of years before and after the book came out. In any case, I'd say that if you enjoyed Madoka you'll probably like this too.
