9 Border does not present any new storyline or concept to its viewers. Its essentially a female centric drama with elements of both romance and slice-of-life. However, with a blend of well known casts, the drama succeeds in presenting a work which is fun and engaging (though not at all times).
It mostly revolves around the three Oba sisters, whose problems we are introduced to at the beginning of the drama. With every episode we see them trying to confront their problems, coming to terms with it and changing themselves for the better while solving it. They grow as individuals for sure, but their bond as sisters also strengthens/deepens. Of course there is romance, but it's not the main part of the story. I personally loved the story of the youngest sister, Hatsumi. I felt that her storyline was something new (everything from the way her character was designed to be and her approach or attitude towards her problems).
I don't think any of the cast members were actors I haven't seen. They are all considerably known to regular J drama viewers and their acting did not fall short, it was well matched to what I think their characters asked of them. Finally, the music was by Sekai no Owari, so I do not have any complains. The rewatch value is as low as it is, because I deem it to be a very good one-time watch, not something to watch multiple times.

Overall, the story is nothing new, but has just enough elements to still keep it fresh among other dramas of the same genre.