I really don't know anything about Mary Kay other than it's a very well-known makeup brand in the US and that this brand is not sold in stores, but through home parties. I didn't get this through a home party, but guess where? Of course, Ebay!
This mascara is cheap too (on Ebay at least) compared to the YSL mascara and yes, I would go as far as to say this is a better mascara. Why? It builds both length and volume like the YSL, but because the brush is rubber, it separates the lashes - which the YSL doesn't do a very good job of doing.
When you first get a tube of the YSL mascara, it doesn't get really good until you've used it a couple of weeks. AND it dries up incredibly fast! I will go through a mascara in 6 weeks and that to me isn't a good deal for that kind of money.
After wearing this for a week I got struck by panic that it would get sold out or discontinued, so I got some back-up mascaras ;)
If you are still on the hunt for THE mascara, please give this a try! I promise
you will love it!!