I first watched Aikatsu! when I was 8 years old and I just rewatched the first episode, so my review will take both points of view in account.
To get rid of the bad stuff first, the characters are veeery boring, in the first episode at least. I remember hating the little brother as a kid and that has not changed. His voice acting is annoying too, like most of the cast (With the exception of Aoi). Plus, the characters are very flat: I had watched most of the anime, but I still only remembered the main trio...
However, the things that go well in this anime is the animation and sound design. Sure, it's bright colors with unicolor-based character designs, but it still made me remember Aikatsu!'s aesthetic. Plus, I was really expecting the 3D animation during the songs to age badly, but I was pleastly surprised! The texturing of the hair and the animation is so smooth, I wouldn't mind an Aikatsu! video game with the same models.
Overall, it's your basic idol anime, but it still has good things going for it. It's worth checking out if you want to pass a good time.