In a nut shell the X ships in the M class have Pod systems that propell the ship. Since their inception they have been a number of cruises canceled because they have broken down. One was canceled at sea. Recently there have been no canceled while sailing but there have been the odd one where ports have been missed or time in certain places has been shortened.
Here's a list of what has happened.
Millennium Inaugural date June 17, 2000
Pod failure dry dock January, 2001
Pod failure dry dock July 16, 2003
Pod failure dry dock December 10, 2006
Pod failure on New Years 2007 cruise and again in January. (Note: ship continued to sail at reduced speed until dry dock in mid April 2007).
Infinity Inaugural date March 3, 2001
Pod failure dry dock late 2001 (3 weeks)
Pod failure dry dock April 13, 2002
Pod failure dry dock January 29, 2003
Pod failure dry dock March 2004
Pod failure dry dock March 27, 2005
Pod failure Ports skipped and cruise shortened on two cruises
(July 9 and July 16, 2006) Electrical problem in pods.
Pod failure dry dock September 13, 2006
Pod failure November 19, 2006. (Note: this ship continued to sail with altered Itineraries and some missed ports until dry dock on May 20, 2007).
Summit Inaugural date October 1, 2001
Pod failure dry dock March 2002
Pod failure dry dock July 18, 2003
Pod failure dry dock September 2004
Pod failure dry dock June 3, 2005
Pod failure dry dock May 20, 2006
Constellation Inaugural date May 1, 2002
Pod failure dry dock May 1, 2002 (salt water leaked into pods at launch time).
I recently got off the Infinity which dry docked in MAy. The ride was great. I was able while aboard to speak with an engineer at length about the problems. They feel that the manufacturer has improved the odds BUT he said. "It's mechanical" we can't say anything for sure.