This will just be a brief review to kind of collect my thoughts about Kin-iro Mosaic.
Light on content (a la K-On or Tamako Market), thin on plot, and heavy on high-pitched moe and friendship, Kin-iro Mosaic is a fairly average show. There's no overarching plot except developing the girls' friendships, but even that seems light and full of silly jokes.
I think my biggest complaint though, is that I never really believed that Alice and Karen were from Great Britain. Sure they had blonde hair, but they never really acted like Europeans. I can't criticize this point too much, because the show never really aimed to be realistic, but, for me personally, it just jarred with my sense of the characters and made the show a little less enjoyable. (Obviously, it wasn't too bad since I finished it instead of dropping it).
I think if you like cute shows about cute girls doing cute things, and don't really care about plots or deep stories, or just want a light, entertaining, silly show to watch, then you'll really enjoy Kin-iro Mosaic.