My family and I went to Portifino's on Voyager last year. I had read, on this forum, that people should not pay the extra as all the food should be of a high standard. The truth is, it is not of a high standard but Portofino's does go that extra half a yard, to make it special.
Is it worth it? My answer would be yes. Should Portofino's be the standard for RCCL, again, I would say yes. However the standard of the main dining room is a good ways short of Portofino's - food wise anyways. The service is excellent wherever you choose.
Have a great time.