It's a joke anime that's taking itself surprisingly seriously, while still being funny through its own excess. It's about keijo, a fictional all-female sport where you fight with your boobs and bottom. Silly? Yup. However it was also one of the biggest hits of the Autumn 2016 anime season, with most of the anime critics watching the show actually being female.
In fact it's a two-layered show. The fanservice is just the surface layer, albeit admittedly attention-grabbing. Yes, the premise is ludicrous. Yes, it's all about girls in swimsuits throwing around their curviest bits. Avoid this show if you think you'd have a sense of humour failure at the idea of epic arse battles. (It is mostly arses, by the way. If you're trying to knock someone into the water, your bottom will usually be a more useful weapon than your boobs.) However:
(a) there's only a very little nudity and (I believe) no nipples even in the Blu-ray versions. It's not that kind of fanservice.
(b) it's being de-sexualised. No matter how deliberately over-the-top the content, it's always being shot and framed like any other sports anime. Baseball, boxing, cycling... they're just athletes at work. Boobs and bottoms are their weapons.
(c) the show's accepting of different body types. Admittedly the artists' idea of an A-cup isn't yours or mine, but even so this is a sport where you're allowed to be huge, tiny, brick-shaped, made of iron or marshmallow-like. You're allowed to have an arse that clenches into a scary steel tendon knot. Every girl has her own fighting style and everyone supports each other. (Well, except for Suruga school and even they're good people really. They're just bringing too much attitude to the annual tournament that's called the East-West War.)
Under that surface layer, it's a straight sports anime.
It's good. I enjoyed it. The cast are likeable, they're passionate about keijo and they're working really hard to become professionals. It's a solid, well-made sports series. That said, though, underneath that's all it is, really. It's a sports anime. It's done well. It won't change your life. You'll know from the starting line-up announcement which teams are going to win the three rounds of the East-West War. (Admittedly the show does pretty well at keeping the adrenaline and tension alive, but I'd written down my predictions and they all came true. There are dramatic rules about this kind of thing.)
This show didn't gain a fanbase because it's great or special. It's not. People watched it because it's warm, fun and a laugh. It has some more meaningful episodes where the girls find a way of helping one of their number address her issues. (Yes, through keijo.) It also benefits from the paradoxical feminism of exploitative anime aimed at sad male otaku, which I've discussed before. There are entire genres where male characters barely even exist and don't get mentioned. Such shows tend to be full of unstoppable, amazing women who can drive tanks, rebuild spaceships and conquer the world. If you can overlook the odd quirk (e.g. fanservice), these can be the most empowering shows ever and you'd want to show them to your daughters as they grew up.
It also helps that the writers have worked hard to invent wacky keijo combat techniques. Boob hypnosis! Nipple full shoulder throw! A swimsuit-shredding arse attack! These are usually ludicrous, but we weren't expecting realism. This show's capable of having you laughing aloud at a topless attack that's also a genuinely clever combat move. (Exhibitionism aside, I'd be surprised if some enterprising fangirls haven't tried playing keijo for real. However I also wouldn't expect it to take off, because real-life keijo would be far more drab and one-dimensional than what we see here.)
It's not flawless, obviously. Those "LOSER!" cries when a contestant falls overboard struck me as a tad harsh. Less trivially, also, I have trouble with Aoba's tendency to give her opponents a mid-battle lecture on everything she's deduced about them. It's an anime-ism. These monologues give away Aoba's own secret weapon (her analytical skills) while making those inferences almost worthless because she's letting in her opponents on her thought processes.
Don't expect too much of this show. It's a well-told sports story that might also make you laugh a lot if you're in the right mood. No more, no less. Keijo school is tough. The girls are sympathetic. A few of them have a bit more depth than the others. Plus of course it's full of silly boob-and-arse battles.
Thinking about it, the show's greatest balancing act might have simply been not getting the tone wrong.