It's hard to say. I DON'T think we would see a crack epidemic, mainly because amongst those people who are likely to smoke crack, the fact that it's illegal probably isn't much of a deterrent. I have a hard time thinking that anyone new would start using now that they could do it without going to jail.
Where I DO think it would have a big impact is on the distribution networks. Decriminalizing crack would cause a massive drop in the premium charged as a street-price, so there'd be less money to be made by traffickers and a drop in the supply provided (since you need huge payouts to justify the risk of being a crack dealer). Could be by a significant amount, too.