I have exchanged to the Dominican Republic twice through RCI. My best advice when exchanging is to double check the all-inclusive costs. A lot of Caribbean resorts have a mandatory all inclusive cost that might end up costing more than just buying a vacation package. I am not sure if it is such a good deal to exchange with RCI into Mexico and Caribbean because of the mandatory all inclusive charges that so many resorts have. I think that Hawaii is a better option now - seems like way more availability too.
Good luck!