Having sailed with Oceania three times before, and finding no reason to change to another cruise line, we booked the Marina after hearing others say it was just bigger, but well spread out. With that I must disagree. Now, do not get me wrong, we had a wonderful cruise, saw wonderful sights, were in good company aboard ship and absolutely loved Red Ginger (would eat there every night if we could), but would never, ever sail on a ship this size again. All our previous cruises had been on Regatta, Insignia, and Nautica. The 680 size is just right. Going to 1200+ is not my cup of tea. The format of the ships is the same, and the addition of the two restaurants is nice, but dining buffet is nothing short of a "cattle call". Bumping and jostling in Terraces was not acceptable. The general climate was more "pay your money, get your goods" than when on the smaller ships. On the latter, you get to know the crew much better, whether that be better or worse, but it definitely gave a more "family" feeling. On Marina it was all there for you, but just seemed colder in feel.
Understand, we were with friends, and I speak of the ship "feel" and the sizing. If you are OK with that then Marina/Riviera might be just fine for you, especially if you are used to 1500-4000 person cruises. Food is very good on Oceania, rooms are nice (beds comfy). Negatives would be exorbitant wine prices. Seeing California wines priced many, many times what you see at home gives you the feeling that Oceania is working your wallet hard. Internet is slow, but it is everywhere at sea. Excursions are overpriced but we use locals interspersed with some Oceania trips.
We like the main dining room as giving the most consistent quality that we look for in cruising and have used it breakfast and dinner very often with Oceania. Problem on Marina was that we had to leave one breakfast, after a 20 minute wait, without breakfast, to get to our tour on time. Sorry, for us it was just not the same as the smaller ships. My guess is that this size may be more economical for Oceania, and if it is, they lose us as customers.