Blue hair is always quite complicated and usually I got stuck with somewhat mushy-green hair after a while.
The color with this is very very nice (if you go light blond before) but with my a bit longer than shoulder length hair, I could only use it once before it was empty, and I didn't manage to get all of my hair colored. So I am stuck with some very very nice blue hair and some that is rather blond.
And as this is rather expensive (plus the shipping) I won't be able to purchase enough for my whole hair (that would be more than 50 euros for 1-2 weeks of blue hair).
Plus this is very liquid, so it feels like it is already thinned out with conditioner a lot.
However, I LOVE the colour, so I am very ambivalent about it