In the right proportions, it turns the paint to the consistency of curdled milk. Pretty nasty actually, and even worse to spray. That's the normal model acrylics (GW, P3, Reaper), not the other stuff like Tamiya acrylics.
I really have not had much success with the new GW base colors, too thick. By the time I thinned it enough to be workable, the color was breaking. I still prefer the Tamiya paints, which you do thin with alcohol. They are really consistent for me, but the color range is somewhat limited. I have some of the Minitaire paints, better color range and pre-thinned, but haven't tried them out yet.