You can't cash in the promo chips, you can only lose them, but you can keep any 'real' chips you win. On the cruise from which we just returned, we had about $280 worth of OBC to use up. Unless, as in our case, you have non-refundable onboard credit to burn. Lots of fun stuff to do on the ships, but we've come to the conclusion that the casinos are not among them on Celebrity.
It only serves as 'seed money' to try to win over that amount. Then there's the 'Free Play' for the tables, which we incredulously found out last time was a case of not ever being able to get that money back. I don't expect to win, but I do think I should be able to play a little while rather than the money being a one-way street. Basically, the only way you 'get that money back' is to lose it, which does not take very long on the Celebrity ships as they have the tightest slots and video poker I've ever come across.