I've been doing the Early Saver since they came out with it. It offers the greatest price protection. Now, the deposit is non-refundable, but you can change your cruise for a $50pp admin fee and they will keep your deposit on file for a length of time should you not be able to go.
There are two types of OBC's, refundable and non-refundable. Jim is correct (sound familiar
) in that the Early Saver OBC's are non-refundable. Meaning use it or lose it. In a case where they are are refundable and you don't use it all they will cut you a check back for whats left.
If you book using the Early Saver rate and the price goes down contact your travel agent. If you are outside of penalty and have not paid in full the difference will be a price reduction. If you are inside of penalty and fully paid then the difference will be given in OBC's.
I "think" you can use them for anything. I don't know about the casino though as I don't use my S&S account for the casino. I keep my gambling funds separate. I learned that the hard way.