Having sailed on both the QM2 and Oceania, our opinion is quite different from some of the others posted here.
To put it bluntly, we wouldn't cruise again on the QM2 if they were paying us to take the voyage. Too stuffy, too many dress code formalities, crew that could care less about you, food in the Britannia Restaurant that was no better than any mass market cruise that could be booked for considerably less money. Public venues closed to you on nights with more formal dress requirements . A class system that should have died when the British Empire died.
Yes, they can have some very interesting lecturers, but that's not a big enough attraction to us to overcome the negatives.
To each his own.