What this is: a pre-makeup base that smooths and mattifies. YSL claims that this will make foundation smooth on easier and will cut down on daytime shine. Matt Touch also claims that the ingredient Ampli Light+ will amplify and diffuse ambient light to give a glowing look and help small imperfections disappear. In the tube, Matt Touch is a translucent white gel-cream. As it spreads on, it becomes invisible with no white trace. This has the usual YSL makeup fragrance - sort of cucumbery floral.
I only use this as a liquid powder for mid-day shine touch-ups and it is AMAZING. I mean, it's perfection in a tube! My skin is generally combination with some shine and larger pores in the t-zone and drier skin/some fine lines elsewhere. By lunchtime my makeup is getting a bit shiny and I need more than blotting sheets to keep me going the rest of the day. My problem is that even the sheerest powders seem to make my makeup look heavier and my cheeks drier by accentuating my fine lines. This is *totally* different. It just erases the shine and minimizes my pores without looking like I added a speck of makeup. I can also totally control where I apply it.
Here's how I use it: I blot my skin of any excess oil. I then blend a grain of rice sized amount between my fingertips so the primer thins out evenly. Then I gently pat my fingertips on all my shiny areas. The key here is to pat, not swipe, a very thin amount over makeup, otherwise your makeup can be disturbed. For my skin type, this gives me a matte finish, which lasts a few hours and develops into a satiny glow by late afternoon.
I think this is expensive at $42/1oz but a tube should last me a very long time the way I use it.
EDIT 11/28/2009: Hm, I found another good use for this primer! I use this as a mixing medium for powder mineral foundation. I start with a damp foundation brush and a small amount of this primer blended into the bristles, then I work the brush into a small amount of mineral makeup (in a small ramekin or bowl). This makes a gel-cream foundation that can be easily brushed on without a powdery finish. :)