So how many user notes do you have?

6 !!!

What do you usually say in a usernote?

I haven't left any. I'll probably leave some uninsightful message or a compliment when I do.

What is the function of usernotes?

I think my friend coined a term that can be used to describe usernotes well enough:

SOCIAL MASTURBATION

What do people usually say about you in a usernote?

I don't know yet. One guy told me to get AWF.
The next person responded to him. - EDIT - Wait, I'm wrong. They were all about me.
But even still, I have no great expectations of being the main subject of any usernotes in my profile.

What constitutes as a good/bad usernote?

A good usernote would make me happy.
A bad usernote would probably some kind of revelation about some horrible thing somebody has done to me in real life.