I absolutely love mac lipsticks with a matte finish but i do find that they tend to dry my lips out a lot. and no one likes a dry and cracked lip. If any of you have used Ruby Woo from mac, you will know what I am talking about. The girl at the mac counter recommended this as it would fill lines and help with the wearability of my lipsticks.
I am extremely happy that i purchased this. once I apply my lipstick over this, it applies smoothly and you can immediately see a difference. I no longer have cracked or dry lips. The lines in my lips are also filled. It also helps if you have problems with your lipstick transferring onto everything your lips touches. Since I use this with matte lipsticks, i don't usually even think about that but i am sure if you had to wear this under any other finish it would come in handy.
The primer has no color but I do find that it amplifies the color of some of my lipsticks. Sometimes when i forget to put a lip balm into my bag, I use a little of this on my lips and it instantly moisturizes it.