MAC's website describe this powder as:
A silky finishing powder that provides an invisible way to set makeup. Reduces shine while optically minimising the look of pores, lines, imperfections. Available in one universal colour that suits all shades. Wear over makeup or on bare, moisturised skin.
I agree with everything it promises. I bought it mainly to help my make up last longer as I felt my oily/combination skin meant that when it came to remove my make up at the end of the day I would only see the last bit of mascara left so I knew I needed to take action. It definitely works to keep my make up on longer but the really brilliant bonus to that is that this powder definitely almost has a 'blurring' effect, it makes your skin seem smoother and almost flawless.
The bottom line is... I love this stuff! It lasts a decent amount of time for the price but you could reserve it for either days that you will have your make up on longer, you're having your photo taken or its a special occasion / event.