I've used it a couple of times. I've used the Dark Neutral Brown shade and the Darkest Brown. What I liked about it was that both times, they came out just that, dark browns and not blacks, like many other home hair dyes can. Also, because you apply it to the roots first (I suppose you pay extra attention to this part as you are following the instructions) I achieve better root coverage.
The only let down is exactly what StepAway said, it doesn't last long enough, but in saying that, not many permanent hair colours seem to on my hair!
I'm the lightest cool blonde shade and I think it's brilliant.... Doesn't dry out hair too much, easy to use, even results good conditioner. Have only had positive reactions and went the same colour as expected I don't have greys yet so can't comment on the coverage of those, but I do have pretty dark roots. My only bugbear is that you have to reuse the gloves after taking them off, so I usually keep another pair handy for the second application.