I don't think the ones that are defaulted on the keyboard work. You have to type them in a special way. I remember there was an article here on wpcentral about them.
I haven't tried most of them though. I normally use a Twitter app. And just barely learned what emoji was when my sister tweeted a picture of a piece of dog crap using the emoji keyboard on her iPhone.
I asked what the **** that was,and she explained emoji and then my cousin joined in talking about it.
I tweeted some emoji icons back, but I couldn't find a way to tweet a picture of the dog crap.
They then made fun of me and said maybe I should quit using Windows Pgone and switch to iPhone.
I replied, "So, you're telling that if I want a piece of crap, then I should get an iPhone?"
Their response.. "touche'..."