Contractions

Question for you all. What do you think about contractions? No, I don't mean the kind that follow pregnancy, I mean the type in literature. You know, like this- don't, won't shouldn't, it's, I'm. Those kinds of things. 
When you're reading, what do you prefer? Do you prefer reading shortened words or would you prefer them to look more formal like this- do not, will not, should not, it is, I am? 
In school we learned not to professionally write with contractions but in works of creativity there is a lot of "rule bending" we do that is acceptable, yet there still seems to be a bit of controversy surrounding the subject. Some people claim writing with full words can make it sound bland and boring while others say contractions can sound sloppy. 
So, what do you prefer? Do you like formal or do you like loose? Let me know in the comments below! And if you head over to my Twitter page, @EvanBondAuthor, you can take part in a poll and see who is right!