I popped into GW in Winchester today and they were doing trials of the new contrast paints. As a slow painter I thought I'd give them a go, the opportunity to base coat and shade in one fell swoop seemed too good to miss!
I quite like them although some are not quite opaque enough, perhaps I failed to heed the warning to shake thoroughly. They are super watery like a wash, so you have to be careful not to splatter them everywhere. It is also quite easy to over apply and then it pools in recesses, but a dry brush or corner of a tissue should be enough to deal with it. They have two new undercoats (wraithbone on the primaris and grey seer on the poxwalker) which give different tones to the colour. You could also use a white undercoat.
I will probably get a couple of them to speed up my painting!