It's a lot better than I'd expected. There are lots of anime series where girls are anthropomorphised ships, planes, guns, swords, etc. This one, though, builds a proper SF framework to explain the existence of its plane-girls, then puts them in an alien invasion story.
The Zai are attacking Earth. They're almost unstoppable and they're wiping out the human race. (In ep.9, the US military have built a Zai simulator that you can play again and again. You win by delaying mankind's extinction for ten years. Our hero, Kei, plays it. FAIL FAIL FAIL. Oh, and he's also a refugee from China, which has been completely overwhelmed.) Zai can fly faster and withstand stronger G-forces than a human pilot, while their electronic counter-measures make it almost impossible for sophisticated missiles to hit them.
The only thing that can kill a Zai is usually another Zai. They're called Anima. Scientists with personality issues grew them from shot-down, still-living bits of Zai, which unsurprisingly makes the conventional armed forces afraid of them. They look like girls, but they're twinned with planes, don't distinguish between their two "bodies" and refer to themselves by the plane's name (e.g. JAS 39D Gripen). All the Anima in this show are real planes, incidentally. They're incredibly expensive to maintain and difficult to make, so there are only a few in existence and, for instance, the USA keeps failing with them and only has one.
The series takes all this seriously. The attack in ep.4, for instance, gets weight and build-up. It's exciting. The military operation being planned in ep.6 is realistic, in that it doesn't revolve around our heroes. They're fighter planes. Unique, valuable ones, but just fight planes. They're flying support and protecting the missiles.
There's also some messed-up logic. This show's portrayal of the U.S. military is so cynical that it could have broken a more frivolous show. (Even some of their own people admit that they're being stupid.) I also disagree with Phantom's nihilistic worldview. Yes, it might be true that her own continued existence would make more difference to mankind's survival chances than, say, the lives of everyone in Japan. That said, though, she's taking her argument too far. Accepting such a loss of human and technical resources would be to hand a big advantage to the Zai. Follow that logic even a few times and you'd find that your self-centred caution had fatally undermined your position.
Anyway, Kei's a teenage boy who finds that he can calm an unstable Anima (Gripen). She might be emotionally defective. She underreacts to things, she has something like narcolepsy and she's in danger of being scrapped. She's also lovable, in her odd way. Japan's other three Anima are a loud, smug idiot (Eagle), a backstabber (Phantom) and an empathic reflector who never goes on missions and whose true appearance is unknowable (Viper Zero).
It's a solid show. The SF holds together, the military side is solid and the otaku-pandering is understated. (Gripen has no nudity taboo, for instance, but the show does very little with it, we never see anything and it makes sense for a walking plane.) I believed in the characters. The only thing that didn't work for me was Kei's childhood friend, Minghua, who throws a massive strop in ep.1 on hearing that he's thinking of becoming a pilot. What's annoying is that Kei then hides his activities from her. He calls it a "part-time job", even though he's constantly being summoned to a military airbase by scary people in black cars. He'll promise to take Minghua shopping, then ditch her and run off to Gripen. This happens every time he opens his mouth. By the time he eventually comes clean (ish) in ep.10, I'd long ago lost patience with their relationship. (Besides, Minghua fancies Kei and we know he'll always be with Gripen.)
I'd recommend this one. It's nothing special, but it's pretty good at what it does. The threat's real and not everyone's going to reach the end of the last episode. (Mind you, who's that fourth plane at 17:15 in ep.12? Viper Zero? It couldn't be the dead one, could it?) That hotel in ep.11 is sinister. I also like the characters. I'm hoping for another season.