Our 18th cruise, the first aboard Oceania's Regatta. We prefer smaller ships (under 500 passengers) but she was fine at over 600. Intimate enough, and we'd book on her again for appealing destinations.
Pluses are her service and food. Staff is congenial and promptly helpful, ship is spotless, cabins are nice but not spacious, highly touted beds are very nice but not that big a deal. Meals are excellent, and you can eat as much (or little) as you like.
The Regatta is excellent, but does have its flaws.
There is no feeling of bonding with other passengers, no gathering at cocktail hours, no mixing and smiling. That's not all bad, but we enjoy getting to know those we're sailing with and we do that on smaller ships.
Open seating in the main dining room is the system, and we hate open seating, much preferring getting to know our waiter and his/her helpers. But open seating works okay on the Regatta, with minimal waits. The buffet meals are very popular, but one can be badly hurt there (kidding) by all the rudeness and shoving and such by fellow passengers. Hamburgers on the bbq are uninspired at best. The two "exclusive" restaurant options, available for booking twice on each 14 day voyage to each passenger, are musts. The steak house is outstanding, the Italian is very good.
Shore excursions are well done but a rip-off financially. Much overpriced. Far better to do your own thing, get a taxi and driver to tour you, or ride local transportation (trollies, subways, canal boat taxis, etc.). Example of pricing: A Helsinke harbour tour cruise booked aboard ship is just under $100 per person. Take it on your own and it's much cheaper at under $25 per person. Oceania needs to reevaluate its excursion pricing or increase cruise charges to offset this bloated profit center.
Cocktail pricing is too high, too. $8 for a glass of Chard is a bit much! Another profit center that irritates.
And if you're a smoker you're a leper aboard this ship. Two small areas on the whole ship that allows smoking, one outside in a corner of the pool deck, the other in the corner of a lounge. No smoking on your veranda and the maids look for evidence (be smart and discreet!). Smoking areas are frequented by crew, sometimes leaving little seating for passengers. Good luck if you smoke.
Still, with all our grumbling, the Regatta is a bit cut above most and we recommend her. So there.