Definitely the Oasis-class ships are something to try at least once! They are truly in a class by themselves and unlike any other cruise ship in the world. If you're going when school is in session, there will obviously be a whole lot less kids, which will result in a much more enjoyable experience.
Food is always very subjective on any ship, but personally we found the food in the main dining room to be just okay. It's tough to prepare food for 6000 quests in 4 hours and be able to do so with great quality. It's not bad, but not as good as RCCL use to be years ago, for sure. The specialty restaurants are very good and while they do have an extra cost, we found it worth it to do them on a few nights.
But these ships are truly a destination in and of themselves. Just make sure you go see all the shows in the main show lounge, in the Promenade, in the ice rink, and in the Aqua Theater. Unlike most shows on ships, we thoroughly enjoyed all of these.