I've used it a few times as a mask. Applied it with a cotton ball, left it on for about 20 minutes and then rinsed it off. I did notice that my skin felt smoother and it didn't break me out at all. ( I have very sensitive skin)
I've seen it mentioned in magazine articles that it helps clear up blackheads and absorbs oiliness.
One reccomendation was to apply it before bedtime and in the morning, rinse off any remaining residue.
Some people apply a light layer of it to their faces as a "primer" prior to putting on foundation. This helps to keep a matte looking finish to your skin and greatly reduces shine.
A friend of mine who battles with very oily skin said it works wonderfully and she doesnt have to touch up her foundation or powder all day. After washing her face, she applies a very thin layer of the milk of magnesia with one of those makeup wedge sponges, let's it dry for a few minutes, then she puts on her concealer and foundation and then sets it all with loose powder. She says that her foundation seems to go on smoother too and looks more natural.