I really love the natural brushes in Photoshop CS5, but the mixer brush leaves a bit to be desired. It appears to imitate oil paint, but without opacity controls it's impossible to emulate oil painting. To adjust the opacity of the paint when oil painting a painter would just add oil to the paint to thin the pigment. Digitally that would be accomplished by just adjusting opacity. It's perfect for gouache painters, however. Absolutely everything else about the brushes are perfect, but an opacity control would make the tool a gem to use when painting for me as a former oil painter. At present I just end up using the natural brushes with the regular brush tool because I can directly control the opacity of my brush.