We stayed at Six Senses Ninh Van Bay for three nights during our honeymoon travelling Vietnam. We booked it as our 'luxury' part of the honeymoon and it certainly did not disappoint. We booked into one of the Hill Top Villas, which was simply sensational accommodation. The privacy and views were without parallel - the villa was huge both outside and inside with its own pool.
What really sets this place apart is the staff. They will go out of their way to make sure your stay is comfortable and enjoyable. All staff apart from the managers, however. We came across the resort manager a few times whose job, it seemed, was to shmooze the guests. We had three separate conversations with him, each time repeating exactly the same thing (where had we been, where are we going, did we like the 'concept'). The insincerity and impersonal nature of these conversations was really off putting. He also glossed over our glowing praise of the staff on our departure, referring to them as 'resources' who got 'excellent training', seemingly praising only himself for their excellence.
The only other off putting part of the stay was the cost of various things. The 'experiences', in my opinion, were fairly priced. The drink, however, was not. I would suggest bringing your own from the mainland.
I would recommend staying here for a few days - it was a truly spectacular setting. However, be aware you will probably spend a lot on food, drink, etc whilst you are there.