I like either the "just made it" or "almost made it" scenarios. They're the most suspenseful endings.

I like it when after she pees in her pants a friend or a bf notices that she is humiliated, embarrassed, etc and thinks what it would be like if I were in their shoes and having not make it and go on the spot and the friend or bf attempts to comfort her make her feel better   

Well, if by happy ending, you mean the girl makes it to the bathroom, I'm indifferent.

Otherwise...  Well, I'm STILL indifferent.

This is quite a dilemma. Happy-ending story and sad-ending story each has different feeling. If the character wets herself and gets humiliated, I may feel sorry for her, but if she makes it in time, I may be little disappointed to not see the wetting. The feeling also largely depends on whether I like that character or not.

Another possible scene is the semi-happy ending. For example, she makes it to toilet in time but leaks a little bit, or she pees in public place to avoid wetting herself, or she wets herself but no one knows it.

It depends on how you define happy ending. I prefer stories where the girl in the end does wet herself, but does get some form of pleasure from it herself (even if she feels embarrassed). On the other hand, if she does make it to the bathroom, the relief can be pretty pleasurable as well of course :)