I'm a repeat Celebrity cruiser sailing five cruises with them mostly on the M Class. I have never been disappointed.
But Holland has a 30-day Hawaii/Tahiti cruise that looks wonderful. Even if the service or the food is not as good as Celebrity, the itinerary makes up for it. Holland does have some unique itineraries that the other lines do not offer.
I've heard the newer Holland ships are quite nice. I guess when I run out of itineraries with Celebrity, I would try Holland.
There is always something you will like and dislike on a cruise. But my feeling is, there is no such thing as a bad cruise.