I bought a job lot of Friends Lego, as I have not built any of the sets. Overall the sets are awful, the animals dont look proper, the figures are far to girly and a lot of the sets dont seem to have a function. (Sort of Barbie like.) It will appeal to a 6 year old, but she will quickly grow out of it and never want to return to it. With boys lego, you can keep it and still play with it when you are 13.
I do like the colours in Friends sets though, just a pity everything has to be Disneyworld-like. Imagine Potter style houses in Friends colours that would appeal to girls and they might want to keep the sets.