My cat Pfeffa does the same thing to me (she's been doing this since she was a little 8-week old kitten...she's almost 8 years old now).
Lately it's when I'm sleeping—she steps onto my chest and puts a leg on each side of my neck and licks my chin, my lips, and especially, my ears.
The whole time she is doing this, she is kneading around and on my neck and head (very painful, even with clipped nails) and occasionally I get a paw on my face as she "assists" me in allowing her to reach the opposite ear. She purrs and purrs the whole time, and really seems to lose herself in the moment.
I have always allowed her to do this for a little while, as it does seem to give her such comfort and pleasure. When it gets to be too much, I push her off and she *usually* will accept this and settle into the crook of my arm. The only thing that helps my neck, is wearing turtlenecks, or bringing the blanket up and around my neck. I also lightly hold her legs while she's kneading to try and prevent any big scratches and to avoid having my face stepped on.