I know of only one case where a guy represented by his Mom signed (in more that 50 drafts). If self represented, most (90%) of the time they wont sign. If the player has a professional agent working for him, he actually probably will sign, but he will also demand a big increase over his orginal demand.
Personally, I just love the idea of some one going to college rather than signing. How much do you figure a degree is worth, because it isn't worth seven figures in your pocket right now.