The dual currency system is fascinating. When I was there a few years ago almost all of the local business exclusively took CUP, although a few vegetable or meat markets would accept CUC and give CUP in change.
As an American tourist (with a French bank account), we were restricted to taking out CUC exclusively at ATMs, and most tourist-facing places would accept only CUC from us (though a few of the paladares and casa particulares preferred USD). The National Museum for the Arts in Havana, for instance, charged the same price in CUC as CUP but only accepted CUC from us (25x the "locals rate").
As an American tourist (with a French bank account), we were restricted to taking out CUC exclusively at ATMs, and most tourist-facing places would accept only CUC from us (though a few of the paladares and casa particulares preferred USD). The National Museum for the Arts in Havana, for instance, charged the same price in CUC as CUP but only accepted CUC from us (25x the "locals rate").