The Riviera is a wonderful ship. Our Veranda-Concierge-Level-Cabin was spacious, well made with finest materials and very quiet. Outstanding comfort.
The most important thing you'll notice aboard is the great crew-passenger-ratio and how spacious the vessel is. It never feels crowded, there was always free space poolside, at the gym and in the restaurants.
Talking about the restaurants: The quality of the food is outstanding - in the regular restaurants as well as in the speciality restaurants. There is nothing more to say about it. Just great.
Another advantage is the average passenger age. I'd say it's well above 60. Which made us (age 30) the youngsters aboard. And that was perfect for me, since I don't cruise to meet new friends. I just want to relax and enjoy the ship, my wife and the destinations. I don't want any loud, party loving teens and twens bullying at the buffet. I really prefer these well mannered, educated seniors. You can have delightful and interesting conversations, but you don't have to. We loved it!
The only disadvantages I can think of are all about the money:
The cruise is quite expansive from the beginning. But with a piece of luck and some patience you might find a good deal - espacially if you can book in Euro instead of Dollars.
They charge a lot for alcohol, but it is not a big deal. They don't push it, you'll be perfectly fine ordering soft drinks and non-alcoholic cocktails (included) only. I spent the incentive onboard credit on a bavarage package and didn't worry at all. After the cruise I discovered in the FAQ that you're actually allowed to bring three bottles of wine from the port aboard, as long as you drink them in your stateroom. That should cover some nice hours on your private veranda.
The shore excursions offered by oceania are of course way too expansive. Frankly I don't get why anyone would book them at all - and why you would complain afterwards. It's really not that hard to organize an interesting and safe trip on your own, for a fraction of the money you'd spend on an official tour.
Last but not least: The internet-fees are shockingly high. But on some itineraries they offer free internet packages. And to be honest: I enjoyed 10 days without internet, email, twitter and Facebook. Just put your iphone in the safe and forget about the outside world. That's a great vacation experience, try it. And: You'll probably find a free wifi at some bar or shop during a shore excursion.
After one month back home I already booked the next cruise with Oceania. It's fantastic!