Which one? There's different mixing mediums that do different things. I have the one for eyes, the water-soluable one. I only use it for pigments, it works well to make it go on more opaque but it's hard to blend out since it's wet. I also find it stings a bit when I put it on, but the stinging goes away when it dries. I know other people have had similar results when I read that on makeupalley.com
I've heard Visine eyedroppers make a good alternative.