I would be very surprised if anyone is disappointed. Celebrity are 'Premium' and RCI are 'standard' (mass market). The 'Solstice' class ships are breathtaking!
Many aspects of the experience should be higher in quality than RCI. The food should be better (more fancy) which is great, unless you are exclusively an egg and chips fan. Although RCI are more party/family orientated, Celebrity are simply more 'upmarket'. For example, you don't get continous annoncement over the PA. Celebrity assume that their guests can actually read their daily news sheets.