It wasn't because of the music, because the music is just bland idol music that's completely forgettable (but there was a noted increase in quality as the show progressed). It wasn't really because of the costumes, or the performances, or anything like that, though they were quite well animated and nothing really to complain about, they did the genre justice.
It was because this was probably the most accurate depiction of performing, and being a performer, that I've ever seen. And it also somehow managed to be a quasi-harem anime with absolutely zero (or very, very close to zero) subtext or ecchi, which is quite an accomplishment.
I have performed some in my life, and sometimes in front of a LOT of people (I've performed in front of more people than probably 99% of you), and it really nailed the atmosphere before, during, and even after a performance. The nervousness, the excitement, all of that. I found myself reminiscing about the past, and even being a little unexpectedly emotional, because I'd actually forgotten a lot of that, and this brought it back.
And not only that, the idol events, etc., were *scarily* realistic. Love! Live, for as much as I loved that anime, was not realistic at all. This was realistic. In the variety show episode, for example, one the games they played was *exactly* like a game they played on an early episode of AKBingo. I actually felt like I was getting a glimpse behind the scenes of real idol culture (and industry, unfortunately), it was that true to form. It actually kind of made me appreciate them more. I mostly don't like idol music, I think idols are trite, untalented, can't sing and dance with any skill most of the time, and generally make their living by dancing around imperfectly and being cheerful. This anime did nothing at all to dissuade me of that notion, but it did implant the notion that maybe there is some value to smiling and being cheerful as your "schtick", as it were. Their talent isn't really one that I respect, but it's a talent, nonetheless, and maybe I shouldn't be quite as dismissive of it. I mean, I could never be cute like that, right?
(the fact that I'm a middle aged male notwithstanding, of course. I probably couldn't have been that cute if I'd been born a Japanese girl with Akimoto Yasushi as a father. It's something, anyway.)
The characters were a mixed bag. Some were well developed, and you started to care about them as the show progressed. Some were just one-dimensional moe-blobs (rock-girl and cat-ears, for example, though their fights were *legendary*). I guess with fifteen girls, though, if they'd tried to develop all of them equally, you'd just end up with no development at all. And the "baddie" in the second season was played perfectly, I think. You couldn't help but hate her, even though she didn't actually do anything evil or even wrong. It was just a difference in how she sees the world. I wonder if that's a comment on the evil stepsisters?
There's a lot of symbolism to the Cinderella story spread across both seasons, but it didn't become clear until the very end, and it was kind of an amazing moment when that happened. But this is a no-spoiler review, so you'll have to see for yourself.
Oh, one other thing: Even though some of the outfits are a little skimpy in an idol kind of way, at the end of the day, there is zero fanservice and zero ecchi in this anime, at all. Not even swimsuits. Even the Producer (who never gets a name) seems to regard them as his... well, daughters isn't the right word... his kouhai, or maybe employees (but not quite, his relationship is a little less distant than that), and he only ever treats them as such. In fact, his inability to even address them in casual form is played for laughs, and I think that's very refreshing. (and if you don't understand Japanese, that will go right over your head, the different forms are untranslatable, mostly.) And they spent quite a bit of time on some rather, admittedly, gorgeous shoes, and that really just makes me want to hand in my man card even saying that. Shouganai. But it's funny, how seeing those kind of mature shoes on seventeen (or younger) year old feet is actually a bit, well... disconcerting. Not in an ecchi way, but, I wonder if that is a bit of what they were going for. Oh well.
That kind of alludes to what there *is* though: Where there isn't ecchi, there's fashion, and a lot of it, some of it kind of ecchi-adjacent. It's only ever treated as fashion, though, so... guess it could be lots worse. Or better, depending on if you like ecchi.
It is actually rather interesting how much this seems like a harem anime, but it isn't, and not even close. You have the producer, a strong, bland male character, and a gaggle of girls who hang around him and do their thing. He is very much the center of their professional world, much as a protagonist is the center of a harem's world. Is there a such thing as an entirely professional harem? Because it really kinda does feel that way. But it was a lot of fun how all the girls interacted with him and each other, they really did behave much like a CGDCT harem does, but, well, without the ecchi. That's a little odd, but very refreshing. They're all just doing their cute girl thing, and that was one of the things that gave this anime its personality.
In summary, if you want an anime with lots of idol music and dancing and cuteness, that nonetheless has a compelling story and treats the characters as real people instead of moe-blobs (most of the time), you'll enjoy this a lot. If you hate idols and the very idea of seeing them as anything but semihuman moe-blobs, you'll hate it. Simple as that. I liked it. Recommend from me.