I am a huge fan of the Army painter warpaints. However, I will not ever use the primers again. They are great for coverage, but you have to really be careful using them. There is no room for error as they cannot be stripped as far as I know.
There is a small window of distance that they need to be used, I think 8 inches or so. Too close and the paint is massively thick. I screwed up big time on a ton of marines, figured I would strip them, and got a bunch of melted plastic. The guys at Army Painter were great in responding to questions about it. they said that it isn't meant to be stripped and to definitely be diligent about the directions.
So take that as a fair warning. I think there are plenty of people that used it and with good results.
I do not wish to detract from this company. I love the washes and warpaints, I just found the primers to be unforgiving. I've actually stopped trying to do fast yellow for my marines. If I could do it all over again though, I would prime white and do several layers of yellow wash. That wouldn't fit into my current army, so I just do the layer build up from Sunset/Yellow Brown up to yellow on a white primer.
Actually I just found this thread on stripping it in case you screw up like me, I think I will give this a go:
http://www.dakkadakka.com/dakkaforum/posts/list/536559.page