I think slang in dialogue, or even in the descriptions (normally fits better if the writing is in first person) can add a lot to the voice of the character. I personally don't want to read about people that sound like robots.
Words I find myself using often in dialogue:
Gonna
Wanna
Shoulda
Things like that, where the word is cut off the way that I normally hear it being said. Not sure if that really counts as slang or not, but you get the idea.
So instead of " I am going to go to the store." I end up with "I'm gonna go to the store."
Do you think this adds or takes away from the characters?
