I personally loved Tsukiani, but I know that not many people would really like this sort of anime.
There's no overarching plot in this anime. It mainly follows the characters' lives and the small challenges they face as an idol. There's not much of them singing or dancing, but it revolves around how the other work that idols go through. So if you're looking for a lot of singing and dancing, this isn't the one for you.
The music is really good. The opening switches back and forth between the sibling/rival units, and the ending features a different characters' solo song each episode. Some people say that
the CGI wasn't all that great, but I don't think so. This anime's CGI is better than most of the CGI I've seen in most idol animes.
The art is pretty standard. It looks nice but it's not that nice. There's not really much to comment on other than that.
What I liked the most about this anime are the characters. The characters are really cute. They're all really lovable and make you want to get to know them, but be warned: there's not much character development in this series. If you're looking for character development, you'd have to listen to the Tsukiuta CD dramas. However, not many of you would take the time of day to read the translations. So unless you are willing to know more about them, you should read them.
It's a nice anime to watch when you want to take a break from all the fighting and drama you see in other animes. It's good to watch if you just want to relax.