We were on Constellation this summer and went to the White Nights ball. For what it's worth, we were bored silly and kept waiting for it to get better - it never did and we left. Then, figuring it had to be us, we tried the "experience" again a few nights later. That's when we knew for sure it wasn't us - it was about as entertaining as watching grass grow.
The original plan was to have Cirque on all four M-class ships but according to what we were told by one the Cirque performers, passenger response has been luke warm at best so Celebrity has opted not to introduce it on Millenium or Infinity.
Not sure which of the "suits" at the home office made the decision to bring Cirque onto Celebrity ships, but judging from the responses over the months on this board and others, I'm willing to bet he/she has had a lot of explaining to do.