THIS BRUSH IS AMAZING!!! I just started using liquid foundation (I have always used MAC studio fix powder foundation) and I found that the regular MAC foundation brush left streaks all over my face and it took forever to make it look decent.... This brush on the other hand makes me look airbrushed and perfect in less that 20 seconds. I just dot the foundation on my face with my fingers and then use this brush in circular motions. Low price, awesome quality!
EDIT: While I still think this brush is amazing, I don't reach for it as much anymore for foundation. I recently purchased the Sigma F80 and it blows this one out of the water. The RT Stippling Brush gives a sheerer coverage while the F80 helps to give a fuller coverage. I still use this one, but usually for cream blush.