We have been on Celebrity, but not on the M Class. We had a wonderful cruise, but we did not do it the way most others did, so not sure how the experience would have been if we had,
The only negative issue was embarkation. They had everyone lined up with only ONE person manning the check-in station. Very frustrating. However, after we passed that hurdle, the rest went quickly.
Also, we did not eat in the dining room except for one breakfast, and to be honest, it was not the best breakfast we have ever had. It took a long time, and was cool by the time we were served. After that, we ate all our breakfasts at the cafe, and they were excellent.
Cabin was one of the "sweet 16" towards the fore of the ship, which have an extra long balcony. We used this quite often, even when it was chilly, because we were able to view Whales and Dolphins. Pure magic!Only problem was the long walk from the front of the ship to the dining in the back, but at least we got some much-needed exercise!