I just returned from sailing of the Diamond Princess. We did personal choice dining and enjoyed the experience. There is the International dining room (Traditional seated dining), Sterling (American steakhouse), Vivaldi's (Italian), Pacific Moon (Asian), and the Santa Fe (Mexican). We made a reservation at each one of the themed restaurants to sample the cuisine. One night for a special treat and a $20.00 additional charge (per person) we went to Sabatini’s trattoria.
No matter which restaurant you dine at, you are also given the "traditional" menu to select from the International dining room. At each of the "themed" restaurants, there are usually two or three choices from that particular restaurant. For example, at the Sterling there was sirloin steak, filet mignon, and some sort of pork chop.
Sabatini's was an elegant and fun dining experience. Instead of selecting an appetizer, entree, etc..., we were given a small amount of everything!
Pick which restaurants you wish to eat in and make reservations immediately. Even if you have personal choice dining you won't be able to just "show up" at a busy restaurant and get in - especially if you want to go to the Princess Theatre.
One couple we spoke with made their reservations too late, but enjoyed the experience because they were given a cozy window table at the same restaurant each evening.