Like it? or Loathe it?
A lot of modern books are increasingly using slang, and more and more slang is being added to the dictionary. So what are your opinions? Have you stuffed your stories with so much slang that it reads like rap lyrics? or do you avoid them like the plague?
I think it depends on how authentic you can make it sound. Sometimes slang reads really poorly and sounds contrived, and I always assume that's where the person writing it doesn't know it well enough.
If it's familiar enough to you that you can make it sound natural, go for it; it can add a wonderful dimension to a book and help the reader feel as though they're fully immersed in the story. If you find that you're guessing how every third word would be written and when you read it back it sounds dreadful, then maybe don't do it.
I think it depends on your character - is your character the sort of person who would use a lot of slang? If so, then you need to use at least a little -if someone from Brooklyn is speaking the Queen's English, it's not going to be too convincing! - just be careful not to make it impossible for the reader the understand!
That's what I think anyway!