As someone who is generally pretty skeptic of a lot of "dark fantasy" anime stories, Muv-Luv did a great job of easing me into it's world with it's initial Rom-com setting of Muv-Luv, moving into the much more grim apocalyptic sci-fi setting of Unlimited and eventually leading into its conclusion with Alternative.

Honestly there's not much to say without spoilers, but Muv-Luv is probably the best visual novel experience I've had and I would highly recommend.
The creator of Attack on Titan himself has stated Muv-Luv to be his main source of inspiration and it lives up to that reputation and more.

The 12 hour rom-com part might not be appealing to everyone, but for those who can enjoy it, it's a lot of fun. For those who can't, I advise to bear with it and at least complete Meiya or Sumika's route once. (Meiya is best girl)