Having sailed many cruise ships and cruise lines, my experience has been that overall there is very little difference in the food, service and entertainment. To say that one line is superior over another cruise line, based on this criteria, is nothing more than one's personal opinion. Some of the newer ships from Princess, Celebrity, Carnival, RCL, NCL, HAL... IMHO, could be rated as premium. The word "premium" is kicked around a lot and while it may make some feel better to say that they sail a "premium" cruise line, my opinion is that in the mass market cruise segment there are premium ships within any given cruise line but not all of the that cruise lines ships are premium. Every cruise line will have its fans that will claim their ship is the best in these categories but at the end of the day it is like the old debate about which is better Coke or Pepsi.
I would put at the top of my list for qualifying as a premium ship would be reliability of the ship when compared to other ships and good customer service when there is a problem. At this time, saying that M-class ships are premium or in other words a cut above other mass market cruise ships IMHO, makes no logical sense at all. Given the recurring problems on the Millennium, Infinity and Summit for the past nine years these particular ships are, IMHO are not premium ships.
Now, after the massive refurbishment announced by Celebrity and IF, and that is a big IF, Celebrity does finally make a permanent fix to the recurring propulsion problems then IMHO, these ships might fairly be classified as a premium ship.
As I have said before I like the size of the M-class ships and thought the main show room was as nice as any that I have seen on any cruise line. The service, food and entertainment were also as good as many other cruise lines that I have sailed.