This was our first Celebrity cruise, having previously sailed with Norwegian (NCL) three times and MSC once. Comparison is inevitable, so I will make a few remarks first but please be aware that this is my opinion and different people have different needs.
MSC is a lower-priced, lower standard, 3-star offer. We enjoyed the Red Sea itinerary they offered and the food and service were less good that either NCL or Celebrity.
NCL attracts lots of families with children and many younger guests than Celebrity. Their prices are slightly cheaper and their cabins slightly smaller but the food offer is on a par with Celebrity. NCL's Jewel class ships (incuding Jade and Pearl which we sailed on) were built by the same shipbuilder (Meyer Werft) as Celebrity's Solstice class ships (including Eclipse) and both share similar high standards of bathroom accommodation inluding a decent size of glass door shower cubicle and no nasty plastic curtain!