This game is incredible from start to finish. I usually stay as far away from visual novels as I possibly can but Vanillaware mixes things up to keep it fresh. There are so many incredibly beautiful backgrounds that will have you forgoing your current objective to sit back and admire the detail and art style for a bit (Amiguchi's room for example).
Though the backgrounds are perfect and the story is an excellent mixture of every sci-fi element you can imagine, the true magic of this game is in the characters. The excellently written and voice acted cast will make you care about their friendships, memories, and struggles. Without spoiling anything, there's a point late in the game where the reasoning for many of their motivations becomes clearer, and their histories are well defined in a pretty mind-blowing way. This adds even more depth to an already complex web of character relationships.
Speaking of complexity, the story is insanely complex and will require lots of attention to detail to really follow it. Luckily, you can open up a dialogue screen to review any recent conversation at any time. There is also a way to view all events in chronological order to clear up any confusion due to the story's non-linear nature. This coupled with a database of terms and characters that you can reference at any time helped tremendously. The most trouble that I had was in remembering the names of the large cast of characters, but I'm also just horrible with names in general.
The combat in this game is so-so, but it's still pretty fun. The story is just so engrossing that it was often hard to shift my attention to combat in order to progress. My advice is to finish as much story as possible before doing the battle missions each time.
The ending really wraps things up but you'll likely be kept guessing about certain things up until that point, so don't stress if it's not all coming together by the time you're nearing completion.