I used the John Frieda one and the first time I dyed it, it turned out very patchy. This could be down to my really thick, long hair and the fact that I was dying it red (it's harder to get the colour to stick seemingly) or it could be that it is just not very good. I used to dye my hair dark brown with a normal L'oreal dye and it would turn out lovely. I did redye my hair the next day and it's better but i'm going to dye it again in a few weeks.
So to sum up... if I could start again, I would go for a normal dye.
I tried the John Frieda one when it first came out and whilst it was easy to apply, after one wash I could see grey hairs so it washed out, they also don't have it in Black so I had to buy Dark Brown.
I then tried L'Oreal Sublime Mousse, not as easy to apply as the John Frieda and it doesn't foam up as much, but I got much better, lasting grey coverage and in the colour I wanted.